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UBC Theses and Dissertations

Interactions and coexistence of species in pasture community evolution Aarssen, Lonnie William 1983

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INTERACTIONS AND  COEXISTENCE OF SPECIES  IN PASTURE COMMUNITY  EVOLUTION  by LONNIE WILLIAM B . S c . ( H o n s . ) , The U n i v e r s i t y  A THESIS SUBMITTED  AARSSEN o f W e s t e r n O n t a r i o , 1978  IN PARTIAL FULFILMENT OF  THE REQUIREMENTS FOR THE DEGREE DOCTOR OF  OF  PHILOSOPHY  in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF BOTANY  We  accept this to  thesis  the r e q u i r e d  as c o n f o r m i n g standard  THE UNIVERSITY OF BRITISH COLUMBIA January  1983  (c) L o n n i e W i l l i a m  Aarssen  In p r e s e n t i n g requirements  this thesis  f u l f i l m e n t of the  f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y  of  British  it  freely available  Columbia,  I agree that f o r reference  agree t h a t p e r m i s s i o n for  in partial  the Library  shall  and study.  I  that  copying o r p u b l i c a t i o n  f i n a n c i a l gain  Department  of  BOTANY  The U n i v e r s i t y o f B r i t i s h 1956 Main Mall V a n c o u v e r , Canada V6T 1Y3  (3/81)  of this  It is thesis  s h a l l n o t be a l l o w e d w i t h o u t my  permission.  DE-6  thesis  s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my  understood  Date  further  f o r extensive copying o f t h i s  d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . for  make  January 16, 1983  Columbia  written  "IT IS GENERALLY ACCEPTED THAT IN ORDER TO COEXIST MORE THAN TRANSIENTLY SPECIES MUST DIFFER - THEY MUST SHOW NICHE SEPARATION. I F SPECIES ARE TOO SIMILAR ALL BUT ONE WILL BE ELIMINATED IN COMPETITION " NEWMAN  (1982)  " I F NOTHING IN BIOLOGY HAS MEANING EXCEPT IN THE LIGHT OF EVOLUTION AND I F EVOLUTION IS ABOUT INDIVIDUALS AND THEIR DESCENDENTS - I . E . FITNESS - WE SHOULD NOT EXPECT TO REACH ANY DEPTH OF UNDERSTANDING FROM STUDIES THAT ARE BASED AT THE LEVEL OF THE SUPER' INDIVIDUAL., •. . . ., - WHAT. WE SEE AS - 'THE ORGANIZED BEHAVIOUR OF SYSTEMS IS THE RESULT OF THE FATE OF INDIVIDUALS" '. HARPER  (1977a)  ABSTRACT  Recent  studies  differentiation  in  plants  response  to  selection  mechanisms  common  biotic  assumption  generally  does  T h i s problem species  trend  of  not  a developmental showed  Temporal p a t t e r n s using  of  essentially  attributes  fields.  of  were  r e s p o n s e s between c l o n e s  For  e a c h of  s p e c i e s were u s e d  in  the a  cover.  species  was  associations in in nature,  the  changes  and  pastures.  which  community  a  persisted  basis  of  evolution to  the  and  a  which  selective  interactions.  experiments  t o one  showed  species  temporary  formed  temporal  and  pastures.  the  associations  be  niche  in older pastures  Interspecific  pasture  can  for  surveys  with  the  for plants.  aged  -amongst  in  The  patterns  a s s o c i a t i o n between  This - data  from b i o t i c  immediate p r o x i m i t y  cover  correlation  more  within-community  Competition reciprocal  had  model  forces accruing  different  relationship  sampling.  unchanged.  qualitative  of v e g e t a t i o n  were p r e d o m i n a n t l y  o l d e r . communities  in nature  e m p i r i c a l support  'constancy'  but  undetermined.  of n a t u r a l s e l e c t i o n  percentage  fine-scale  specialization  competition),  largely  three  little  contact  younger communities  (e.g.  i n a study in  with  intraspecific  coexistence  have s t r o n g  time-series  analyses  studied  processes  i n c r e a s i n g community  suggested Soil  by  species  i s addressed  of  associated  i n v o l v e d remain  interactions  Ordination  demonstrated  interactions  that  explained  divergence  have  another  set of as  3 pastures, diallel  up  for  individuals  investigating w h i c h were i n  a c t u a l neighbours five  design  of and  t h e most three  in  the  abundant different  i v  species  pairs  Results  suggested  species  may  direction  of  Samples of  each  with  increasing  Lolium  a  genet  contribution  to  from d i f f e r e n t  ability)  but  natural  pairs  the  did  response  from  had  the  yield  beneficence)  may  ability'  (balanced  and  in  the  balanced  exclusion. collected  i n the  i n the  oldest  presence  of  design.  Natural  most  equitable  component  (i.e.  highest  combining  yield  and  from p a i r s of  2)  non-  abilities  of  resources  on  which they  both  discussed  in relation  reduces  the  of  of  ability ability  found  for  differential  in  s u p e r i o r components f o r  make demands.  These  theoretical  findings  are  considerations  i n s y s t e m s of c o m p e t i t i o n .  coexistence  between t h e  ability.  and  types  combining was  in  neighbours  combining  Most e v i d e n c e  to contemporary  e v o l u t i o n a r y theory  competitive  1) e c o l o g i c a l  inferior  operating  selection  alternative  competitive  competitive  proposed d i s t i n c t i o n  two  abilities).  selection  that  i n t e r a c t i o n s among  in  selection  natural  suggest  other  in plants:  competitive  this  studies  mechanism,  and  either more  localities  t o grow  in t o t a l  result  (niche d i f f e r e n t i a t i o n ) ,  general  particular  repens c l o n e s  transplant  differ  these  to competition  'combining  of  age  design.  neighbours.  Results  In  ability  total  not  between  competitive  Trifolium  reciprocal  neighbouring  2).  towards  p e r e n n e and pairs  relations  series  differentiation,  or  were t e s t e d f o r t h e i r in  a replacement  increasing pasture  niche  abilities,  other  (e.g.  in  that competitive  neighbouring  pasture  studied  change  competitive  as  were  concepts  i s o f f e r e d b a s e d on of  fundamental  A a  niche  V  TABLE OF  CONTENTS  ABSTRACT  i i i  TABLE OF CONTENTS  v  L I S T OF TABLES  viii  L I S T OF FIGURES  xi  ACKNOWLEDGEMENTS  CHAPTER  1.  GENERAL  xiv  INTRODUCTION  1  PROBLEM TERMS AND  2 CONCEPTS  .  A) INTERACTION AND COEXISTENCE  9 9  B) FUNDAMENTAL NICHE REQUIREMENTS  11  C) RELATIVE COMPETITIVE A B I L I T Y  14  Offensive Characteristics Competitive A b i l i t y  Conferring 15  Defensive C h a r a c t e r i s t i c s Competitive A b i l i t y  Conferring 18  PROGRAMME  CHAPTER  2.  20  THE STUDY S I T E : VARIABLES AND  INTRODUCTION  PATTERNS  ........  ..22 23  THE GEOLOGY  24  THE CLIMATE  29  THE SPECIES  30  A) PATTERNS IN THE VEGATATION  31  Methods  32  Results  33  B) COLLECTION AND  PROPAGATION  OF CLONES  42  vi  SOIL SAMPLING  AND ANALYSES  45  Methods  45  Results  46  DISCUSSION  55  CHAPTER 3. A QUALITATIVE MODEL FOR PASTURE COMMUNITY EVOLUTION: PRELIMINARY EVIDENCE FOR SPECIES INTERACTIONS AND COEXISTENCE USING CONTACT SAMPLING  64  INTRODUCTION  65  METHODS  69  Vegetation  Sampling  69  Data A n a l y s i s  71  RESULTS  75  DISCUSSION  93  CHAPTER 4.  COMPETITIVE RELATIONS  IN DIFFERENT AGED  PASTURES: A MIXTURE D I A L L E L OF F I V E SPECIES INTRODUCTION  106 ..107  METHODS  111  RESULTS  114  DISCUSSION  CHAPTER 5. PASTURES:  . . . . . ..... ;  COMPETITIVE RELATIONS  .  .......  IN DIFFERENT AGED  SUBSTITUTIVE REPLACEMENT SERIES MODEL  INTRODUCTION THEORY METHODS RESULTS AND DISCUSSION  . . ... ..V. 126  132 133 135 .140 143  vi i  CHAPTER 6. BIOTIC SPECIALIZATION AT THE GENOTYPE L E V E L : RECIPROCAL PHYTOMETER TRANSPLANTS AMONGST FOUR NATURAL NEIGHBOURING GENET PAIRS OF LOLIUM PERENNE AND TRIFOLIUM REPENS  158  INTRODUCTION  159  METHODS  164  Experimental  Design  164  Data A n a l y s i s  168  RESULTS  171  DISCUSSION  182  CHAPTER 7.  GENERAL DISCUSSION  194  PROSPECTUS  195  COMPONENTS OF A GENERALIZED COEXISTENCE THEORY  197  A) G a u s s i a n  Coexistence  197  B) C o e x i s t e n c e by s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) C) C o e x i s t e n c e combining  by s e l e c t i o n  ability  for competitive  (coevolution)  203  OTHER MECHANISMS FOR COMPETITIVE BALANCE 1) N o n - e v o l u t i o n a r y  210  mechanisms  2) F r e q u e n c y - d e p e n d e n t - s e l e c t i o n v... . .  210 ;  3) D i f f e r e n t . l i m i t i n g f a c t o r s NICHE DIFFERENTIATION VERSUS COEVOLUTION OF COMPETITORS: ALTERNATIVE EVOLUTIONARY SOLUTIONS FOR SPECIES COEXISTENCE CONCLUSIONS  LITERATURE CITED  201  . . ... 211 214  218 229  231  vi i i  L I S T OF  TABLE  1.  TABLES  P o s s i b l e c h a r a c t e r s which confer competitive ability i n p l a n t s i n c o n t e x t s of a) r e l a t i v e exploitation ability, and b) relative interference ability  16  TABLE 2.  Details  23  TABLE 3.  The m o n t h l y d i s t r i b u t i o n s o f a) r a i n f a l l (cm), b) snowfall (cm) and c ) t o t a l p r e c i p i t a t i o n •(cm)  29  The monthly distributions o f : a) mean d a i l y temperature ( C ) , b) mean daily maximum temperature ( C) , c ) mean daily minimum temperature ( C ) ; d) number of d a y s w i t h f r o s t ( d a i l y minimum below 0 C)  30  TABLE 5.  A species l i s t  31  TABLE 6.  Species c o m p o s i t i o n of t h e ' H i g h - l a n d ' f o r a g e m i x t u r e u s e d i n sowing t h e t h r e e p a s t u r e s .  TABLE 4.  on t h e t h r e e  study  f o r the study  fields  site  32  TABLE 7.  Product moment correlation coefficients for t h e 9 measured soil variables with quadrat percentage cover f o r 10 o f t h e most abundant s p e c i e s o v e r a l l at the study site and with axis scores f o r the first 3 axes of the o r d i n a t i o n o f 20 s e l e c t e d quadrats. .. .......... . . .56  TABLE 8.  Simpson's d i v e r s i t y i n d e x e s (D) c a l c u l a t e d f o r e a c h s u r v e y b a s e d on t o t a l p e r c e n t a g e c o v e r of each species per f i e l d  62  TABLE 9.  Count m a t r i x  72  TABLE  10.  Contingency  TABLE  11.  Summary of significant (P<0.05) individual p a i r w i s e a s s o c i a t i o n s i n the three pastures i n each survey  generated table  by c o n t a c t s a m p l i n g  for ( i , j ) interaction  72  76  ix  TABLE  TABLE  TABLE  TABLE  12.  13.  14.  15.  TABLE 16,  TABLE  TABLE  17.  18.  Numbers of significant temporary, s e a s o n a l , and s t a b l e a s s o c i a t i o n s (both positive and n e g a t i v e ) d e t e c t e d i n t h e t h r e e d i f f e r e n t aged pastures , 5 x 5 (means diallel  94  matrices of p e r - s p e c i e s y i e l d s (g) of 3 replicates) from the mixture i n v o l v i n g 5 s p e c i e s p l a n t e d as ramets.  11 5  A summary of s i g n i f i c a n t (P<0.05) t r e n d s i n combining ability indexes (CA) and total yields from t h e m i x t u r e d i a l l e l f o r d i f f e r e n t genet p a i r t y p e s a s t h e age of the pasture from which they were c o l l e c t e d i n c r e a s e s .  1 24  N u t r i e n t c o m p o s i t i o n of p o t t i n g mix used for the competition e x p e r i m e n t between d i f f e r e n t g e n e t s of L o l i u m p e r e n n e and T r i f o l i u m r e p e n s .  . . 1 65  Tabular model variance.  used  the  in  analysis  of 1 70  Y i e l d s (g) o f p h y t o m e t e r s of T r i f o l i u m repens (Tx), each collected with a natural n e i g h b o u r i n g g e n e t o f L o l i u m p e r e n n e (Lx) from f o u r s i t e s i n t h e 1939 p a s t u r e and p l a n t e d i n all combinations of clover type and g r a s s type  1 72  Yields (g) o f p h y t o m e t e r s of L o l i u m p e r e n n e (Lx), each collected with a natural neighbouring genet o f T r i f o l i u r n , r e p e n s (Tx) • from four sites i n the 1939 pasture and p l a n t e d i n a l l c o m b i n a t i o n s of c l o v e r t y p e and grass type  173  T o t a l combined y i e l d s (g) of p h y t o m e t e r s of Lolium perenne (L) and T r i f o l i u m r e p e n s (T) c o l l e c t e d as n a t u r a l n e i g h b o u r i n g pairs from four s i t e s i n t h e 1939 p a s t u r e and p l a n t e d i n a l l combinations of grass type and clover type  1 74  :  TABLE  19.  X  TABLE 20.  Combining a b i l i t y indexes f o r genet pairs of Lolium perenne (L) and T r i f o l i u m r e p e n s (T) c o l l e c t e d as n a t u r a l n e i g h b o u r i n g pairs from four s i t e s i n t h e 1939 p a s t u r e and p l a n t e d i n a l l combinations of grass type and clover type  175  TABLE 2 1 . A summary of interpretations of competitive relations between species r e p o r t e d from t h e experimental i n v e s t i g a t i o n s .  208  xi  L I S T OF  FIGURE  1.  FIGURE 2.  Contour  FIGURES  maps of t h e s t u d y  fields.  25  Total percentage cover of the 14 most abundant s p e c i e s p r e s e n t i n the t h r e e p a s t u r e communities over the 33-month sampling period  34  The 10 most a b u n d a n t s p e c i e s p r e s e n t i n e a c h study field based on mean p e r c e n t a g e c o v e r over the study p e r i o d .  40  FIGURE 4.  Ordination results  43  FIGURE 5.  Mean values for chemical from 20 s e l e c t e d s t a n d s from 23 and 42 y e a r s o l d )  FIGURE 3.  FIGURE 6.  FIGURE 7.  FIGURE 8.  FIGURE 9.  FIGURE  10.  o f t h e 27 s u r v e y s a n a l y s e s of s o i l e a c h p a s t u r e (4,  47  Distributional patterns of the 9 soil v a r i a b l e s on t h e a x i s 2 v e r s u s a x i s 1 plane of the stand ordination of 20 selected q u a d r a t s from e a c h pasture.  49  Stable pairwise associations (positive s o l i d l i n e s , and negative broken lines) detected i n a) t h e 1958 p a s t u r e , and b) t h e 1939 p a s t u r e  86  Course lines showing the degree of association for selected individual species pairs versus t h e number o f y e a r s s i n c e t h e s a m p l e d f i e l d was sown  88  A qualitative evolution  98  model  for  p a s t u r e community  Planting arrangement i n the mixture d i a l l e l s h o w i n g p o s i t i o n s f o r 25 r a m e t s of each of two spec i e s  112  xi i  FIGURE  FIGURE  FIGURE  11.  12.  13.  I n d i v i d u a l y i e l d s , t o t a l y i e l d s and c o m b i n i n g a b i l i t y i n d e x e s (CA) f o r e a c h g e n e t p a i r t y p e c o l l e c t e d from d i f f e r e n t aged p a s t u r e s ( 0 , 2, 21 and 40 y e a r s ) and grown i n c o m p e t i t i o n i n a mixture d i a l l e l  118  Ratio diagrams illustrating 5 possible outcomes f r o m a r e p l a c e m e n t s e r i e s e x p e r i m e n t reflecting 5 d i f f e r e n t types of c o m p e t i t i v e r e l a t i o n s h i p between s p e c i e s i a n d j .  136  Planting arrangement the r e p l a c e m e n t s e r i e s  141  f o r 13 ramets i n  FIGURE  14.  Ratio  f o r the experimental r e s u l t s .  ..144  FIGURE  15.  R a t i o d i a g r a m s showing t h e o r e t i c a l t r e n d s f o r e v o l u t i o n a r y changes i n c o m p e t i t i v e r e l a t i o n s between two species (i and j) during different stages ( 1 , 2 and 3) o f n a t u r a l select ion.  148  R a t i o d i a g r a m s f o r i m m e d i a t e l y a d j a c e n t genet pairs from t h e t h r e e d i f f e r e n t aged p a s t u r e s (2, 21 and 40 y r s ) s u p e r i m p o s e d on t h e same graph  151  Planting arrangement f o r pots different genet combinations p e r e n n e ( L ) and T r i f o l ium r e p e n s  containing of Lolium (T)  166  Yields o f phytometers of L o l i u m perenne (L) and T r i f o l i u m r e p e n s (T) when grown t o g e t h e r in d i f f e r e n t c o m b i n a t i o n s of genet types  176  Combining a b i l i t y indexes and t o t a l yields for different genet type combinations of L o l ium p e r e n n e and T r i f o l i u m r e p e n s  179  Relationship between total yield and combining a b i l i t y index f o r d i f f e r e n t genet type combinations of L o l i u m perenne and T r i f o l i u m repens  183  FIGURE  FIGURE  FIGURE  FIGURE  16.  17.  18.  19.  FIGURE 20.  diagrams  used  XI 1 1  FIGURE 21.  The schematic s t r u c t u r e of a g e n e r a l t h e o r y of species coexistence in contexts where competition i s an i m p o r t a n t f o r c e of n a t u r a l selection  1 98  xiv  ACKNOWLEDGEMENTS  I am most d e e p l y g r a t e f u l gracious this  Roy  s u p p o r t and e x t e n s i v e e n t h u s i a s m  research.  attentive  Much d e b t  and  Appreciation Neill  t o Dr.  i s owed t o D r .  invaluable  counsel  i s expressed t o Dr.  for, i n s p i r i n g  in  Turkington  i n the s u p e r v i s i o n of Piet  de J o n g  forh i s  statistical  J a c k Maze and  discussions  forh i s  to  and f o r t h e i r  matters. Dr.  W.E.  interest  i n my  from  Brenda  research. I wish Kilgren, Julie  to  acknowledge  Melanie  Madill,  technical Carol  assistance  George,  Diane  Downum, A n g e l a Chen, and E l e n a K l e i n e .  Helene  Contant  foreign  literature.  Botany  Garden  glasshouse  Joop  van  I also  Nursery  Velzen  thank  for their  Thanks  i s due  to  forhelp in translating  the  staff  care  of  the  U.B.C.  i n the maintenance of  support through postgraduate s c h o l a r s h i p  S c i e n c e s and E n g i n e e r i n g Research C o u n c i l  gratefully  Jan Evans,  experiments.  Financial Natural  and  Scott,  of  from t h e  Canada  is  acknowlegedv  Finally, Chard  and Ms.  this  research  I  extend  Mary C h a r d on  their  affectionately dedicated.  immeasurable fortheir property,  gratitude  cordial  t o Mr.  welcome  to  a n d t o whom t h i s  William conduct  thesis i s  1  CHAPTER 1  GENERAL  INTRODUCTION  2  PROBLEM  The long  coexistence  of s p e c i e s which  i n t e r e s t e d e c o l o g i s t s and t h e q u e s t i o n  many  competitors  largely 1977,  to  unresolved  coexist  1979).  coupled  with  the concept  defined  as  health  the  The  competitive limited  of  environment. basis  specific  1972a).  ecologies  reduces the p r i n c i p l e perspectives  have  c o n c e r n e d w i t h ..what I s t h e l i m i t competitive  exclusion  regarded  The  MacArthur  similarity  to a ' t r i t e  of niche  &  Wilson  interestingly, the  evenly out  i n numbers (Grinnell  like  have  been  prevent resource  formed  maxim' this  the  perhaps the  of  and  in  identical  (Cole  overlap  1960)  are  more  beyond ] which"  i s . imminent.  ,This  of s p e c i e s p a c k i n g  1972,  principle:  aptly  t o be p r o p o s e d  (MacArthur  MacArthur  exclusion  t h e same f o o d  balanced  the other"  1967,  in a  a s "...  &  Levins  Newman  r e c a p i t u l a t e s one o f t h e e a r l i e s t  competitive  approximately  species'  be  efforts  has  of;one of the s p e c i e s  of  may  common n o t i o n  abandoned  Grubb  h a s been so  mechanisms  ever  more o p e r a t i o n a l view embodies t h e t h e o r y 'limiting  so  e x c l u s i o n ' , much  principle  o r law o f n a t u r e  (Vandermeer  recent  i t  f a c t o r s or  been  ecology"  but  that  one s p e c i e s by a n o t h e r  has  principle  1969,  coexistence  In g e n e r a l ,  Gause's  and  permits  Miller  of c o m p e t i t i v e  e s t a b l i s h i n g what  exclusion  theoretical  of  of c o m p e t i t i o n  i s the absence of d i s e a s e .  concerned with  o f what  1959, 1961,  notion  'absence  c o m p e t i t i v e l y has  i n some c o m m u n i t i e s h a s c o n t i n u e d  (Hutchinson  Werner  only  interact  and  1967,  1982) and  statements  of  "Two  species  of  h a b i t s a r e not l i k e l y  t o remain  long  i n t h e same r e g i o n .  1904) ( i t a l i c s  added).  One  will  crowd  3  Most  of  the  observations extensive niche'  of  with  niche  review  by  animals  development deal  of  support  of  the  the fact  food  Schoener central  niche  concept.  that  niche  in  be  explained  in part  by  the  diversity  of  We  are  still  (Harper  to  Plant  by  their  Willson scarcity  of  Bazzaz  1977,  &  Berendse  response  to  partitioning 1979,  Waser  (Pickett 1977). any  Yet,  real  (Harper  and  competition  depth  1981a,  and  Fitter  in  (Reader  separation  1976,  1978,  of  i t is difficult overlap  along  plants. plant  (1978) have  recent show  of  roots  differences Silvertown  moisture  and  light,  1976,  in  1981),  i m a g i n e how  and  enormous This  gradients  Piatt  & Weis  immobile,  make  there  on  nutrients)  could  not  opportunity has  lack  demands  w a t e r and  to  i n many p l a n t c o m m u n i t i e s .  Some  is  Yeaton et a l .  relatively ,  This  P a r r i s h & Bazzaz. 1 9 7 8 ,  & Piatt  supply (of  1976,  for  s p e c i e s may  1976,  soil  p l a n t s are  energy  animals  importantly  placement  1982),  1975,  Werner  same r e s o u r c e s  niche  comparable  plants.  plant  P a r r i s h & Bazzaz  because h i g h e r  1968),  considerable  1975,  in  'food  explaining  in  co-occurring  of p o l l i n a t o r s  the  more  communities.  .microtopography - (Brattori  choice  essentially  that  1979,  & Bazzaz  nothing  Antonovics  differentiation  example,  1978)  the  c l e a r e m p i r i c a l evidence  in herbaceous p l a n t  shown  see  theoretical  coexistence  (1973) and  particularly  (Wieland  early  p r o b l e m of  niche  for  (e.g.  i d e a of  m o b i l i t y , but  the  from  e c o l o g i s t s however must  The  within-community  differences,  the  1977b).  however w i t h  drawn a t t e n t i o n t o t h e  have  The  comes  f o o d s a v a i l a b l e t o them, e s p e c i a l l y  coexistence.  studies  animals  p l a n t s have v i r t u a l l y  can  true  principle  (1974)).  was  the  species  Gause's  differences  to  left  for  raised  be for  serious  4  doubts concerning  any  predominant  niche  differentiation  as  (Grubb  1977,  & Agren  and  Fagerstrom  The  study  i s at  the  of  species  places from  a mechanism 1979,  It  strong  embodies  a  emphasis on  Pickett i s an  competitive  factors  1977b).  (Harper  the  evolutionary  community  community  selection Darwin  plants  problem  ecology.  It  organization  view'  opposed  of  of  1980).  of  s e l e c t i o n pressures  i n t e r a c t i o n s as more  and  'Darwinian  the  importance  for coexistence  coexistence  i n t e r a c t i o n s and  which emphasizes  in  widespread  i n t e r f a c e of p o p u l a t i o n  concerns population diversity.  or  to a  nature  which  which  accrue  'Wallacian  pressures  r e f e r s to the  and  from  view'  abiotic  p r a c t i c e of  some  making, .... the deeply-seated error of considering the p h y s i c a l c o n d i t i o n s of a c o u n t r y as t h e most important for i t s inhabitants; whereas i t c a n n o t , I t h i n k , be d i s p u t e d t h a t the n a t u r e of the other inhabitants, with which each has t o compete, i s ... generally a f a r more i m p o r t a n t e l e m e n t of s u c c e s s (Darwin 1859, p. 400) .  Bock  (1972) has  emphasized  "constant  modification  species  other  sharp  on  contrast  to the  environment.  mutually  e x c l u s i v e ; the  versa.  the  changes  many  in  of  species  These  two  effect to  vegetation  of  abiotic  biotic  individual  species  this  exerted  selection  that within  result  by  are  s e l e c t i o n pressure  attention  i.e.  by  stands  s e l e c t i o n pressure  a part",  may  that  of  provide  exerted  selection"  types  studies  are  forces"  , and  n a t u r a l environment  which they  interactions  "static  (1982, p.402) draws  in their  vegetation  biotic  selection  highly  response  Pigott  observation  of  interacting  physical  influence  that  to  the  plants the  from,  the  and  each in the not may vice  frequent  "respond  context  that  and  a  of  to the  r e a c t i o n of  give  rise  to,  5  r e s p o n s e s of other that  under  pressures origin with  benign  related  physical  that  Tilman  conditions,  the  uncertainty  interacts  (1978) major  will  selection  be o f b i o t i c  genotypes of the a r e the major  stress  organisms  source  of t h i s  uncertainty.  Central species  t o much o f  coexistence  accommodation process  of  empirical  McNaughton  attempt  organization. in  ( e . g . Sanders adjustment &  Wolf  discussion  accommodation Antonovics  competitive  succession.  the  1968).  (1979)  or  (1978,  p.246)  relations  out  role  writes  have  Even  of  the  community  genetic  been  has  never  studied,  or  during  pertinent  between,  t o any  need  of  convergent  adaptation  been  no  community  s y s t e m s , and i n  the. conflict  biotic  the extent  "...  invasions  Such c h a n g e s a r e v e r y  i s the  that  in  rarely  , during  biological  This  point  its  that  to their  t o a common e n v i r o n m e n t , and d i v e r g e n t  members  view  reflects  of s p e c i e s  of communities as c o e v o l v e d  investigation.... adaptation  is  seems t o have been made t o e s t i m a t e  i n e s t a b l i s h e d communities  ecological  ecology  and community d i v e r s i t y  evolutionary  biological  changes  community  among s p e c i e s  environment.  other  &  the changing  w h i c h an i n d i v i d u a l  either  Glesener  to environmental  and s u g g e s t  biotic  of  species.  to  explicitly  invest igated". To  understand  understand approach basic  this  of b i o l o g i c a l  evolutionary  problem  i t is  mechanisms necessary  components o f t h e q u e s t i o n  generally place  the  the nature  been n e g l e c t e d .  between  for  to  i s to  coexistence.  recognize  the  a t hand, components w h i c h  If competitive  two i n t e r a c t i n g  accommodation  species  exclusion (besides  is  to  To most have take  the requirement  6  that  resources  conditions  be i n s u f f i c i e n t l y  must  exist:  1) t h e i r  number o f n i c h e d i m e n s i o n s beyond  a  superior  certain  comprehensive competitors  must  fundamental  theoretical 1971,  1976,  Levin  1980,  Braakhekke  &  1978,  component  Newman  and B u e l l  of b i o t i c  involved  in  rewards.  E v o l u t i o n of b i o l o g i c a l  species  will  evolution 1971,  differentiation  exerted  Turkington  &  Harper  organization  work  The  community  the  on  in and  into  found  by  a  of the  yield  direct &  view  individual  the  Woodwell  evidence f o r  selection  1969,  rich  interacting  specialized  results  advance  focuses  has  (Watson  d e v e l o p m e n t s have l e d some t o which  &  of the p r o c e s s e s  (Whittaker  to differing  neighbours 1979c).  i n the study  accommodation  Recent  i n response  by d i f f e r e n t  Bergh  component  among o r g a n i s m s s h o u l d  of s i n g l e p o p u l a t i o n s  produced  & McGehee  den  second  that  characteristics  1975).  Futuyma  & F u e n t e s 1975,  van  investigations  community  of community  Whittaker  ecotypes'  communities,  affect  The  (1969) s u g g e s t  interaction  &  none.  evolution  the  widespread  1976, A r m s t r o n g 1977,  1)  competitive  Collwell  1974, C o l l w e l l  1982).  components:  received  (e.g.  A  for potential  relative  has  Christiansen  has r e c e i v e d almost  Langford  2)  1975, Roughgarden  Fenchel  overlap  requirements.  of c o e x i s t e n c e  and  1974, 1976, S c h o e n e r  &  comparison  theory  and e m p i r i c a l a t t e n t i o n  Pianka  Whittaker  first  (or the  i n t e r a c t ) must  common r e s o u r c e  requirements  The  essential  requirements  t h e r e f o r e encompass two d i s t i n c t  niche  abilities.  two  p o i n t , a n d 2) one o f them must be a  f o r these  evolutionary  supply)  resource  i n which they  critical  competitor  limited  'biotic pressures  Linhart  1974,  these  recent  f o r the  plant  of  genotype  as t h e  7  principal 1977b,  unit  of  1982).  community  This  which  are  now  single  coexistence The  and  change  in  and  (see  investigate  important  in  evolutionary  to  community.  the The  concept-oriented not a  the  system  and  available for  in  response the  1) How  to  following do  in  genotypes  evolution  and  under  than  of  the  pasture  systems  i n which  extensive  reviews variation  highly  susceptible  and  biotic  factors  (1978)).  The  of of  plant  aim  biotic  species  of of  the this  interactions  system-oriented.  and  of  investigation  and these  pasture primarily  Objectives  ecology  as  to  are study  interactions is  community  investigating evolutionary  how  the is  neighbour  environment.  to  specialization  evolution  i n t e n s i t y of  biotic  is  and  mechanisms  problem  their  of  where g e n e t i c  i n w h i c h d i f f e r e n t s t a g e s of  are  p r e c i s e manner  r e g a r d e d as  qualities  coexisting  i n which a high  individuals.  i n the  so much t o u n d e r s t a n d more a b o u t p a s t u r e  predicted  on  the  structure  rather  on  thesis  ( 1 9 6 9 ) ) and  Snaydon  to  contribute  the  i n t e r a c t i o n (see  physical  r e v i e w by  amongst  understanding  of  role  this  i s abundant  is  accommodation  in  Risser  to  and  specialization its  Harper  both population  operating  generally  important  species  goal  1976a,  communities.  study  response  environment study  populations  under  of  to c h a r a c t e r i z e biotic  (1963) and  constituent  common  produces  i s an  Donald  objectives  forces  needed  Pastures are  competition  in  a  hence, of  system  community.  by  into  selective  selection  within  d i v e r s i t y (Antonovics  merges t h e  ecology  c o n s e q u e n c e s of Efforts  community  development  changes  in  species  Emphasis  is  centered  questions:  interspecific  competitive  relations  change  as  a  8  community  ages?  2) I s t h e r e  greater  biotic  specialization  in  populations  from  o l d e r communities? 3) I s s p e c i a l i z a t i o n 4)  Does  biotic  genotype  levels?  5) What 6) What  reciprocal  specialization  between occur  i s t h e e v o l u t i o n a r y mechanism role  permitting  does  the  coexistence,  evolution  have  species? a t b o t h t h e s p e c i e s and  of b i o t i c of  i n community  biotic  specialization? specialization,  evolution?  9  TERMS AND  The  terminology  coexistence To  avoid  are  used  concepts the  in this  niche  other,  The  The c h o i c e  same  and  t h e terms  as  they  o f m e a n i n g s f o r terms and  t h e b i a s e s and o b j e c t i v e s  is  true  confusion  i s related  the r e s u l t  follows,  here.  surrounding  to the fact  an  that neither  attempt  is  made  f o r , and a d i s t i n c t i o n  competitive  competition  the concepts  ability  in particular  understood.  elucidate  between,  the In  operational  fundamental  reference  of  t h a t an a t t e m p t t o  i s clearly to  of  I t i s proposed  one i s o f t e n embedded i n a c h a r a c t e r i z a t i o n o f  with  meanings  to define  reflects  the semantic  characterize  what  often  and c o m p e t i t i o n  niche,  i n many v a r i e d ways i n t h e l i t e r a t u r e .  thesis.  i n ecology  that  to  i t i s necessary  practitioner.  however  related  h a s been u s e d  ambiguity  CONCEPTS  n i c h e and  to plants.  A) INTERACTION AND COEXISTENCE If .  two s p e c i e s  i n nature  interact,  this  implies  g e n e r a l : s e n s e t h a t one' ( o r b o t h ) s pe c i e s i n into their  the  sphere  life  cycles.  ecologists mutualism,  of i n f l u e n c e of t h e o t h e r  include  between  repeatedly  (not  periodically)  in  and the  a  between  interaction  members  given  two  If  In p l a n t s  then  during  but  by  parisitism, a  certain  populations  continually  area,  species  two  e n t e r (s)  recognized  predation,  of  way  a t some t i m e  commensalism.  necessarily  as ' c o e x i s t i n g ' .  interaction  of  competition,  amensalism  interaction  regarded  Types  some  i n t h e most  at  occurs least  t h e two s p e c i e s may be  f o r example, a populations  competitive  may n e v e r  occur  10  between v e g e t a t i v e seeds  for  sites suitable  competition neither into to  may  s p h e r e of  limitations  for  imposed by  event  activity In  then in  another.  this  thesis,  the  a l t h o u g h Harper  a  sense,  come  into  be  distinction and/or  resources  scale:  interaction);  and  any  species  of  (or  a  notion implies (or or  i n w h i c h no of  impact  a  at  be  used the  for  i n the  ...  existence  could  continuous  least  in  products)  implies  sharing  above  term c o h a b i t  (1979)  particular one  coexist.  the  'occurring  by  due  determined  not  differences  of  either  organism a f f e c t s  Werner  'coexistence'  enter  is  their  on  If  interaction  struggle  based  the  do and  one  will  way,  or  agents.  continuously  (1961) use  'cooccurrence'  incidence  by  interaction,  interaction  only a mutual presence regarding  genetically  two  'cohabitation'  without  or  possible".  incorporating  above;  i n any  that  terminology  spatial  other  a_l.  proximity  dispersal  the  activity  et  or  between  establishment,  to  "individuals  considered of  equilibrium, detailed  i.e.  such  reasonably  of  and  occur  potential  term c o e x i s t  context, similar  the  the  the  repeatedly  pollinators  i s continuous  of  may  dispersal  then  which  but  germination  influence  requirements,  Coexistence an  occur  for  s p e c i e s however has  the  site  individuals,  of  without  suggests  a  in  temporal  a  stable  interaction habitat  or  continuous  together'  implies  assumption  i s made  organism  on  another.  11  B)  FUNDAMENTAL NICHE REQUIREMENTS The  different  reviewed  by  Whittaker  et a l .  inclusive' Krebs  meanings  several  niche  competitive  ability  has  its earliest  idealized  distribution  interactions was  live  more  'realized'  specific niche  That  competitive  niches  of  According competitive  to  found  in  as  the  nature,  - the  'actual'  potential area  1968, niche  within  the of  of  the  which  actually  is reflected  a  finds in  the  versus  'pre-competitive'  versus  C o l w e l l & Futuyma (MacMahon e t a l not  .  part  out-compete  common  niche  necessarily  1971), 1981). of  the  from  the  stated,  the  i s further evident  species with  which  the  niche as  conception  means t h a t two  exclusion  term  absence  Variously  principle,  the  the  principle.  Gause's  however,  'fundamental'  is  will  basic  1928)  area  view  1957),  of a s p e c i e s n i c h e  one  the  as p a r t of  where one  of  ability  -  species'  1924,  Grinnell's  to the  (Levins  in  (e.g.  including i t s  Where  (Grinnell  conceptions  coexist  (1927) ' a l l -  common usage  species.  This general  exclusion  cannot  1972,  Elton's  individuals  (Hutchinson  essentially  Vandermeer  been r e g a r d e d  species.  competitive  characterization  been  interpretatively  connotation  niche  1968,  have  most c o n s i s t e n t usage of  other  'potential'' versus  principle  ever  1972).  'post-competitive' and  rarely  'niche'  regarded  other used  as o p p o s e d  (Vandermeer  later  with  "pre-interactive  s p e c i e s can it"  with  is  been  The  recognizes  has  'profession'  has  of a s p e c i e s .  niche  and  relationships  concept  niche  or  term  1981).  niche  1979)  'role'  competitive  the  the  (MacArthur  Hurlbert  of  197-9, R i c k l e f s  'position',  authors  1973,  notion  of  very the  similar other.  results  means  in  different  1 2  competitive ability  abilities;  exclusive  have of  represents  environment, question  of  requirements) (e.g.  resource  states  renewability, for The  required  biotic the  (individual of  these  availability  of  niche  of  the  niche  symbionts pollinators species). p.177) :  as or  ability  as  in spite  of  organism This  fundamental  -  niche  with  its  raises  the  niche  and  quantity,  quality,  etc.)  Other  lichens,  affects  individuals  of  site)  and  requirements descendents.  availability  s p e c i e s through  effects  These e f f e c t s  translate  of one  organism  on  (and  obligate the  a recent  themselves are exception  view  gender  from  the  fitness) not  part  of o b l i g a t e  benefactors  opposite  one  influences  in turn a f f e c t  the  of  into  organisms t h e r e f o r e a f f e c t  (with  of  the  sets  intensity,  to leave  that they  or  of  t h a t meet m i n i m a l  the a c t i v i t y  to recognize  fundamental  possible  conditions  requirements  This p a r a l l e l s  all  (or  chemicals,  states.  requirements in  the  as  species  of a n o t h e r .  i t is crucial  'niche'  fundamental niche  to a r e f e r e n c e  resource  but  the  or p o p u l a t i o n )  other  interactions  well-being  of  considered  (e.g.  resources  or more of  be  energy,  activities  of  between  that  periodicity,  a species  competitive  most  in general.  i s proposed here  resources  E v i d e n t l y then,  i t s competitors.  relationship  interaction  It  includes  competitive  E l t o n i a n view) t h a t the  relationships  which  precludes  niche.  idea  ( f o l l o w i n g an  the  the  the  regarded  principal  claims  all  of  implicitly  the  contradictory  niche  categorically  from t h e d e f i n i t i o n  ecologists  biotic  this  such  as  in dioecious  Hurlbert  (1981,  13  As a c o n s e q u e n c e of i t s r e s o u r c e - u s i n g a c t i v i t i e s , an organism h a s many i n f l u e n c e s on the surrounding community or ecosystem. I f the t o t a l i t y of these i n f l u e n c e s or impacts i s regarded as t h e 'functional role' of t h e organism, then ' r o l e ' i s not e q u i v a l e n t t o n i c h e , a s many a u t h o r s would have i t , but r a t h e r ' r o l e ' i s a consequence of the n i c h e . Biotic  interactions  species. between  In p a r t i c u l a r , species.  some r e s o u r c e further, having the  requirements  that by  coexistence generally  each  the  interact  other.  the  This  have a t l e a s t  this  i n common,  availability  two-component  biotic  one  neglected  step  on t h e i r  b u t a l s o on  of  resources  perspective for  interactions  in predator/prey  above a r g u m e n t s , be  and  has  been  and h o s t / p a r a s i t e s y s t e m s  in  systems  ability  of  h a s been  interactive  s e n s e most c l o s e l y  Hutchinson  environmental  and w i l l  (1957),  range  of  but  in  1978).  distinct in this  resembling  resource  competitioninfused  into  a  species  i t i s potentially  of  without  fundamental concept  thesis  from  in a pre-  t h e hyper-volume  requirements  of  primarily  terms  a  that  i . e . a multidimensional  i n d i v i d u a l ) w i t h i n which - "...  as  be d e f i n e d  limitations  descendents  i t i s proposed  regarded  ability  (Hutchinson  overlap  of n i c h e .  requirements  merely  i f they  Taking  (niche)  the n o t i o n of c o m p e t i t i v e  whole  only  common.  on  competitive  the  the n i c h e s of  of organisms depends not o n l y  a p p l i e s to a l l  Following  of  of  a r e consequences of n i c h e  in  has  recognized  precept  niche  consequences  requirements  example, b u t l a r g e l y  because the  Organisms  some r e s o u c e s  impact  are  they  the coexistence  required  for  then  view  d e s c r i p t i o n of  encompassing a l l (population  capable  of  or  leaving  reference to competitors,  requirements  and  tolerances  ..."  14  A  habitat  represents niche the  an  that  of  available  niche  realized  niche  the  term  the  and  deny  species'  living,  fundamental  In p r a c t i c e  or  they  hence d e l i n e a t e s a  the  of  occupation  space,  habitat.  a c o n s e q u e n c e of  word  however,  of  may  certain  expand  the  'post-interactive'  Accordingly,  fundamental niche  biotic  and  realized  interaction.  be  points  Milne  (1961).  between  supply to  variation  one  on  the  'competition'  regards  divided into  adopted  of  resource both  and  was  will  as any  types  -  by  defined in  Birch as  which  other  an each  from  Tinnin  literature. by  Hall  in  A further who  beneficial,  which  the  term  In u n d e r l y i n g mechanisms t o view t h a t c o m p e t i t i o n  1969,  a  (1972)  (1974a)  d e t r i m e n t a l or  context  1967,  of  r e l a t i o n s h i p . between  'exploitation'  Miller  be  of  definition  make demands.  effect,  variety  in reviews  u n i t s to the  confusing  the  a  organisms  t h e more t r a d i t i o n a l two  have  strictest  introduced  another,  B i r c h 1957, here.  the  more)  i s used p r e s e n t l y .  later,  to  ' i n t e r f e r e n c e ' i n the  o r g a n i s m on  1954,  (or  which they  interference  considered  thesis  Competition  variable and  come  following proposals  two  in terminology  'interaction'  (Park  used  the a v a i l a b i l i t y  limited  has  In t h i s  will  interaction reduces  'competition'  in ecology.  (1957) and  be  capable  for occupation.  niche  within  is  RELATIVE COMPETITIVE A B I L I T Y  meanings  be  of  s p e c i e s may  space,  is  species  i s a c o n s e q u e n c e of b i o t i c  The  of  available  available  interaction  a  proportion  of o t h e r  portions  C)  which  actual is  presence  niche  in  and  be can  'interference' -  Berendse  1981b)  will  1 5  An  even  more  'competition' alone •an  is  is  ability  entirely  by)  individuals  resource(s).  Competitive  measure  1)  of:  availability to  tolerate  of  a  first  case  the  second case,  and  making  demands  ability  may  of  'defensive'  relative  to.another. ability  t h e same  t o another,  interference competition by  through uptake to  another.  'offensive' characteristics,  and i n  of  into  a  plant  to  will  two  This  concert  important  c a t e g o r i e s , and d e f i n e 1) r e l a t i v e  of e x p l o i t a t i o n -  the  to resources  the q u a n t i t y  in  reduce the a v a i l a b i l i t y of  ability:  procurement  Ability  characteristics  efficiency  from t h e s i t e .  effective  by_  competitive  reduces the a v a i l a b i l i t y reducing  t o reduce the  characteristics.  competitive  access  combined  and 2) t h e a b i l i t y  availability  interference a b i l i t y  with  limiting  as a  individual  of  may i n  Offensive  - the r e l a t i v e  w i t h i n (and  neighbourhood  on  to  ability  ability  fall  ability  context  only  be r e g a r d e d  an  resource  characteristics  2) r e l a t i v e  solely  a  than  c h a r a c t e r i s t i c of  manifested  p l a n t which c o n f e r  of r e l a t i v e  another  in  i t s overall  competitive  ability  Competitive  prevailing  resources  literature  Character i st i c s C o n f e r r i n g Competitive  determine  types  the  be c o n s i d e r e d  Numerous  resources  in  is  ability  of l i m i t e d  the  Offensive  the  reduction  Attributes  the  ability'.  i t has meaning o n l y  characterized  in  as i t i s n o t an a b s o l u t e  Competitive  interacting  term  'competitive  misleading  individual;  others.  confusing  of  of  resources, level  resources  resources  by  requires attributes of  of  1). E x p l o i t a t i o n  limited  of those  two  exploitation  relative  (Table  to  resources.  to  removal related  Interference  TABLE  1.  Possible characters  in  plants  b)  relative  in contexts  of a)  which c o n f e r relative  interference a b i l i t y .  competitive  exploitation  ability  ability,  and  17  a)  EXPLOITATION COMPETITION  b)  1. Rapid growth r a t e ( i . e . e f f e c t i v e p h o t o s y n t h e t i c metabolism and/or e f f e c t i v e or rapid uptake/storage of n u t r i e n t s and/or water).  INTERFERENCE COMPETITION  1. E n c o u r a g i n g , s h e l t e r i n g o r c a r r y ing p e s t s , d i s e a s e organisms o r p r e d a t o r s which a f f e c t o t h e r s p e c i e s (Janzen 1 9 6 6 , Sandfaer 1968,  2.  3.  T a l l p l a n t h e i g h t , long h y p o c o t y l s stems, p e t i o l e s (Black I 9 6 0 ) . Large  l e a f area  (Black I 9 6 0 ) .  k. Leaf o r i e n t a t i o n f o r e f f e c t i v e c a p t u r e o f 1 i ght .(Mons i et_ aj_. 1973). 5.  Deep and/or e x t e n s i v e root  6.  Large embryonic c a p i t a l s i z e ) (Black 1 9 5 8 ) .  7.  E f f e c t i v e timing (e.g. e a r l y a r r i v a l at a s i t e , e a r l y germina t i o n and/or e a r l y e s t a b l i s h m e n t ) ( e . g . Sagar 1 9 5 9 , Harper 1 9 6 1 ) .  8.  9.  system.  General a d a p t a t i o n , r e s i s t e n c e , o r p l a s t i c i t y to 'adverse' e n v i r o n mental c o n d i t i o n s o t h e r than those imposed by c o m p e t i t o r s (Daubenmire  1970b).  Encouraging/producing rhizosphere components o r s o i l r e a c t i o n s unf a v o u r a b l e to o t h e r s p e c i e s (Trenbath 1 9 7 6 ) .  3. A l t e r i n g the a b i l i t y o f the environment to p r o v i d e s p e c i a l i z e d t r i g g e r mechanisms ( e . g . f o r the breakage o f dormancy (Harper 1964)).  (seed  A t t r a c t i v e flowers, p o l l e n , seeds, or f r u i t s f o r e f f e c t i v e p o l l i n a t i o n and/or d i s p e r s a l (Donald 1 9 6 3 , L e v i n & Anderson 1 9 8 0 , Davidson S Morton 1 9 8 1 ) .  1968).  2.  1970a,  k. D e p o s i t i n g a dense l a y e r o f l i t t e r on the ground s u r f a c e (Grime 1 9 7 3 ) . 5.  R e l e a s i n g substances which a r e di r e c t 1 y t o x i c to o t h e r s p e c i e s ( i . e . a l l e l o p a t h y ) (Rice 197*0.  6.  I n t e r f e r i n g with p o l l i n a t i o n (e.g. " p o l l e n a l l e l o p a t h y " ) (Kanchan 1 9 8 0 ) .  18  competition, resouces  in c o n t r a s t , reduces  (i.e.  without  attributes  directly  competitor  from  that  an  ability  feature"  of  however, not  (de Wit It  ability  the  of  niche  effects  interaction  may  to  plant  involves  preventing  a  argues  is a  "general  From  Table  competitor  1965)  and  1  will  'strongest',  (Harper  from c o m p e t i t i o n  those  (1979, p.20)  plants.  'biggest',  hardships  both  responses  overlap  or  'most this  experiments with  is  plants  only  competition caused  by  reproduction different  by  a  organs,  referred  higher  that or  the  t o as  term  amensalism,  the  proximity  of  by  as an  'amensalism'.  interference i.e.  to  "...  those  neighbours  ...").  u s u a l l y thought  imposed  and  s p e c i e s , such  t o be  evolved  competition  Conferring Competitive  (e.g.  relative  ( f o r example by  year, by  competitive  Ability ability  t o l e r a n c e d u r i n g p e r i o d s when  competition in  one  occur  1979).  In some s p e c i e s , h i g h e r by  limitation,  i n c u r r e d by  and  shortage  Characteristics  limited  resource  phenomena however a r e  Roughgarden  conferred  be  i n t e r f e r e n c e e f f e c t s may  p r e f e r r e d t o use  to resource  1974,  Defensive  that  may  which are  Interference  to note  be more s u i t a b l y  (1961) has  encompass  are  in  'highest y i e l d i n g ' evidence  This  matter production  that a superior  be  i s important  detrimental  (Harper  Grime  of  1 970) .  regardless  Gill  seen  necessarily  by  related  getting a resource.  be  or  accessibility removal).  indirectly  competitive  aggressive',  resource  t o maximize dry  i t can  supported  or  the  dying  varying back  above  number ground  be  resources  d e l a y i n g or  the  may  forgoing  or  size  parts  of but  19  avoiding plants,  total  death).  This w i l l  where t o l e r a n c e a b i l i t y  characteristics underground  as  p a r t s (e.g.  indeterminate  growth  1977b).  Immunity  others  would  conferring  seed as and  competitive  be  will  dormancy,  be  life  type  ability.  of  important  reflection forms w i t h and  construction  allelopathic a  a  in hemicryptophytes modular  to toxic  also  be p a r t i c u l a r l y  substances defensive  of  in  such  sustaining geophytes),  (Harper produced  1964, by  characteristic  20  PROGRAMME  The system  studies of  described  three  development. two-fold:  coexistent  The  1)  experimental  time-series  not  out  periodically  data  for  over  evolution.  empirical  basis  the  three  experienced  identical  or  a t any  grazing) .Quadrat  patterns,  habit  the  The  is  not  experience  of  its  terrestrial  pastures  means t h a t  physical  contacts  plants  a recording between  a quantitative basis  "plant's  experience  each  contributes  pastures  were c a r r i e d  obtain  quantitative  to  aim  features  of  was  obtain  of  reveal  the  however three  (e.g.  gross  sensitive  and  an  are  pastures  of  climate  to  neighbours. the  the  high  number  over  time  vegetational the i n d i v i d u a l The  of  above  this  (Harper  in  ground  a parameter  i n d i v i d u a l experience, diversity"  sessile  sward d e n s i t y  for discussing patterns  community  pasture  development.  to  of  to  assumptions  conditions  If monitored  level of  and  individuals reflects  represents  the  two  developmental r e l a t i o n s h i p  immediate  interaction.  at  the  wholly  fine-scale  coexistence  The  u n l i k e l i h o o d that  serves  it  of  interactions.  principal  s t a g e of  investigation is 2)  developmental  environmental  given  community  patterns;  three  Important  the  a  other.  the  to  on  vegetational  a 33-month p e r i o d  sampling  but  the  centered  pasture  p r o b l e m under  another  s u r v e y s of  communities. concerning  plant's  one  for referring  recognized  of  biotic  characterizing  community  of  stages  a v a i l a b l e from  Percentage cover  t h e s i s are  s u r v e y s of  of  complement  information  this  approach to the  analyses  approaches  in  of  procedure of i.e.  local the  1977b).  21  This  sampling  the  present  by  Yarranton  pastures  study  and  variety  investigate  the  individuals  of  data  in d e t a i l  A  actual  the  series  phytometer  different  biotic  in  the  natural  selection  general  evolutionary  discussed contexts  fields  in of  et  a_l.  1979,  Alternatives in  its versatility have  recently  and been  (in press).  methods  and  (e.g  were  of  designs.  A  at  different  the  and  i n two the  in  are  used  operating theory  relation competition.  to  reciprocal investigate  species  to  collected  pasture ( i . e .  level).  i n t e r p r e t e d by  which e x p l o r e s  This the  in  systems  of  coexistence  other  i s made  i n response  oldest  as  replacement  to  species  evolutionary context. chapter  Use  and  same  the  genotype  results  age.  1978)  another  interactions  f u r t h e r method of  devised  g e n o t y p e s of  t o one  d i f f e r e n c e s in  Trenbath  neighbourhoods  final  w e l l as  i n immediate p r o x i m i t y  specialization  and  studies in  of  transplanting is  a theoretical  in recent  1981).  e_t a l .  in pastures  neighbouring  originally  i n t e r a c t i o n s o c c u r r i n g between c l o n e s  1960)  specialization  length  Jong  designed  Aarssen  sampling  in  to  Where a p p r o p r i a t e , into  as  i s used  employed  diallel  different  1 977,  experimental  i n the  biotic  used  1979b, S a u l e i  de  biotic  Wit  reciprocal  from  by  same s p e c i e s  (de  al.  sampling',  I t was  been  vegetation  w h i c h were  mixture  has  analysis  of  neighbours  the  and et  'contact  purpose.  1979a,  as a method of  presented  for  (1966)  & Harper  methodology  known as  for this  (Turkington  Turkington  value  method,  theories  of  of  placing  them  i s pursued  at  i m p l i c a t i o n s of competition. is  A  proposed  and  coexistence  in  22  CHAPTER 2  THE STUDY S I T E : VARIABLES AND  PATTERNS  23  INTRODUCTION The dairy  study  farm  site  consists  owned  Highway, A l d e r g r o v e Valley The  of B r i t i s h  farm  William  (SW  1/4  Columbia  since  last  ploughed  the  fields  to  as  the  will  the  indicating  40, the  competition to  summarized  TABLE 2.  named a f t e r 21  and  ages of  above  in Table  Details  on  25,  has  2  the  these  the  1930 1900  on  average,  cut 20  the  25704 F r a s e r  in  the  Fraser  122°30' 45"W  The  managed  The and  dates  Long.).  as  grazing  three pastures 1977  were  and h e r e i n a f t e r  or w i l l  be  pastures  referred  (respectively)  when c l o n e s were c o l l e c t e d  fields  were a l s o  recent p l a n t i n g  three  study  used  dates.  as  for  pastures  Details  are  fields.  Age when r e s e a c h commenced (years since l a s t ploughed)  Size  (ha)  2  1 .04  1958  21  1 .44  1939  40  0.99  have a l w a y s been e i t h e r f o r hay.  t o 30  on  2.  1 977  occasionally  10)  been  year-old  fields  most  1 942  pastures  Twp.  1958,  Approx. date of c l e a r i n g ( f r o m scrub/brush) D a t e of and f i r s t last ploughing ploughing and s e e d i n g .  The  Chard,  the c e n t u r y .  i n 1939,  experiments. the  Sec.  located  Mary  (49°03'45"N L a t . ;  of  seeded  be  and  1965), but  turn  and  three pastures  i n the C o a s t a l D o u g l a s - f i r b i o g e o c l i m a t i c  (Rrajina  land  prior  by  is situated  zone of B.C.  of  Grazing  cows p r e s e n t  grazed  in a l l three  intermittently  by  cattle  fields  or  involves  from May  until  24  November.  Weather  December and the  stated  April.  dates,  treatment  The  1939  elevation  are of  None of t h e  fields  have been  ever  received  nor  spread  i n the  Contour  rarely  have  (fertilizers  mechanically  slope  conditions  and  of  the  maps of  contiguous trees.  The  bordered  by  or  pesticides).  g r a z i n g between reseeded any  Barnyard  since  chemical manure  is  periodically.  1977  1977  they  permit  pastures are g e n e r a l l y f l a t  field. site  the  The  varies  fields  (Fig. entire  1958 from  are  drainage  110  shown  1) but site  field to  is  122  from  enclosed  d i t c h e s and  slight  is gently r o l l i n g . m above  in Figure  separated  with a  by  1.  one  The  level. pastures  another  wire  l a n e w a y s , and  sea  The  by  rows  fencing  and  i n some p l a c e s ,  trees.  THE The  general area  sediments deposits The  upon w h i c h of  study  loam,  is  drained. Humic soil  latest  site  i s u n d e r l a i n by  truncated strata  lie  material  specifically  soil  parent  has  is Luvisolic  (down t o t h e  and  the  poor  Committee 'C  of  of  Eocene  glaciomarine  12,000 y e a r s  B.P.).  to f i n e - t e x t u r e d  Humo-Ferric  Survey  t o p of  (11,000 -  moderately  non-stbney.  G l e y s o l (Canada S o i l depth  a  P l e i s t o c e n e age  generally The  GEOLOGY  to  moderately  Podzol  and  1978).  Average  horizon)  i s 80  -  clay well  Orthic top  100.cm.  25  FIGURE given of  1.  Contour  i n m e t e r s above  60 q u a d r a t s  Insets  i n each  maps  of the study  sea l e v e l . field  show t h e p o s i t i o n a l  used  Points in  fields. represent  the  relationship  Elevations are the l o c a t i o n s  vegetation  of the t h r e e  surveys. pastures.  28  29  THE Most c l i m a t i c of  the study  winter  usually  The s t u d y  164.3 cm p e r annum  not c o v e r e d  weeks.  and  site.  were r e c o r d e d area  w i t h heavy p r e c i p i t a t i o n ,  averaging  2  data  CLIMATE at Aldergrove,  features  mostly  a  by snow f o r p e r i o d s l a s t i n g  total precipitation  a r e shown  The m o n t h l y d i s t r i b u t i o n s  snowfall  (cm)  of  east mild  rainfall,  the area i s  longer  than  of r a i n f a l l ,  i n Table  TABLE 3.  of  In w i n t e r  The mean m o n t h l y d i s t r i b u t i o n s  km  relatively  i n t h e form  (1953-1970).  2.4  1 or  snowfall  3.  a)  and c ) t o t a l p r e c i p i t a t i o n  rainfall (cm)  (cm),  b)  (means f o r 1953-  1970, A l d e r g r o v e ) . Jan  Feb  Mar  Apr  18.7  16.5  14.6  10.9 7.2  b) 41.9  12.4  10.4  c)  17.8  15.6  a)  22.9  The easterly from  is  winds  the south  years was  area  1953  June  July  Aug  Sept  6.6  4.3  5.2  9.4  0.3  0.0  0.0  0.0  0.0  10.9 7.2  6.6  4.3  5.2  9.4  0.5  exposed  i n the f a l l  predominantly  and w i n t e r  and s o u t h - w e s t  the  minimum  distributions  temperature  to  and  Nov  Dec  18.5 20.7  22.0  0.0  5.1  25.4  18.5 21.2  24.5  north  months.  in spring  Oct  and  of  winter  summer.  temperature  climatic  and h o u r s of s u n s h i n e  For  was  variables  a r e shown  north-  Winds a r e m o s t l y  t o 1970 t h e maximum summer a i r t e m p e r a t u r e  36?C w h i l e  Monthly  May  i n Table  the  recorded -20  0  related 4.  C. to  30  TABLE  4.  The  temperature mean d a i l y frost  monthly  (°C),  b)  distributions mean  daily  minimum t e m p e r a t u r e  (daily  Aldergrove);  minimum  of h o u r s of  Regional  Atmospheric  1980, P a c i f i c  d)  0 °C)  e) number  a)  mean  maximum t e m p e r a t u r e  (°C);  below  of:  number  of  (means  sunshine  (°C), c)  days  for  (means  Environment  daily  with  1953-1970, for  1970-  Service).  Jan  Feb  Mar  Apr  May  June  July  Auq  Sept  Oct  Nov  Dec  a) 0.8  3.3  4.9  7.6  11.5  14.2  16.1  16.0  13.6  9.3  4.9  2.5  b) 4. 1  7.6  9.8  13.3  17.8 20.2  23.2  22.9 20.3  14.6  8.8  5.7  -0.9  0.0  1 .9  5.2  8.2  8.9  9.1  6.9  4. 1  d) 21  1 5  1 7  9  3  0  0  0  1  5  e) 68  77  11 2  1 64  209  217  291  245  1 74  1 37  c) -2.4  1 . 1 -0.7 1 3  1 7  72  54  THE SPECIES A total the  3  pastures,  fields  were l a s t  Trifolium land' in  sown  mixture  with  a  comm.)  but i n f o r m a t i o n  (Table 6).  mixture  5).  A l l three  comprising  ( T a b l e . 6 ) . . The same s p e c i e s  at a l l three  only  seed  (Table  , 15-20% D a c t y l i s g l o m e r a t a and 70-80%  mixture  available  herbaceous s p e c i e s are present i n  11 o f w h i c h a r e g r a s s e s  repens  forage  this  pers.  of 28 n a t u r a l i z e d  seeding  f o r the mixture  times  on t h e i r used  were  (Richardson  relative  in planting  5-10% 'Highpresent  Seed  Co.,  proportions i s  t h e 1977  pasture  31  TABLE 5.  A species l i s t  f o r the  study  GRASSES Agropyron Aqrostis  NON-GRASSES  repens alba  (L.)  Beauv.  Festuca Holcus  Achillea  L.  Carex  Anthoxanthum odoratum Dactylis  site.  glomerata rubra  L.  L.  Hypochoeris Juncus  L o l i u m mult i f l o r u m Lolium perenne  Lam.  L.  Phleum p r a t e n s e  vulqatum  Cirsiurn arvense  L.  L.  L.  (L.)  Medicaqo  lupulina  Plantaqo  lanceolata  P l a n t a q o major  L.  Ranunculus a c r i s  L.  Poa  trivialis  L.  Rumex a c e t o s e l l a  L.  Rumex c r i s p u s  L.  Rumex o b t u s i f o l i u s Stellaria  the  cover  of  spring  (March),  surveys to  test  similar  two  three  (L.)  Vill.  Taraxacum o f f i c i n a l e  Weber  Trifolium  repens  Trifolium  pratense  pastures  carried  hypotheses: the  was  L.  surveyed  9 o c c a s s i o n s over  t h r e e summer  between  media  L.  L.  VEGETATION  s p e c i e s on  were  L.  L.  L.  E a c h of  L.  sp.  compressa  PATTERNS IN THE  Scop.  radicata  Poa  A)  L.  sp.  Cerastium  L.  lanatus  millefolium  out. 1)  (June) The  youngest  and  percentage  a 33-month p e r i o d .  and  surveys  Species  for  three  fall  (September)  were c o n d u c t e d  composition oldest  is  Three  in order  the  p a s t u r e s ; 2)  least Species  32  composition in  the  i s t h e most v a r i a b l e  youngest  throughout  community and t h e l e a s t  t h e 33-month  variable  period  i n the o l d e s t  community.  TABLE 6.  Species composition  used  sowing  the in  in  1977 m i x t u r e t h e 1977  of t h e ' H i g h - l a n d '  the three p a s t u r e s . only.  forage  mixture  P r o p o r t i o n s shown  are for  L o l i u m p e r e n n e was a t e t r a p l o i d  variety  mixture. Dactylis  glomerata  Tri folium  45%  pratense  20  L o l i u m perenne  15  Lolium multiflorum  10  Phleum p r a t e n s e  5  Trifolium  2  Ladino  repens  Clover  (1977  ( T^ r e p e n s  ) 3  only)  Methods Each  survey.  was  based  systematically  positioned  systematically  arranged  m,  total  giving  results per  field  effort,  were  a l l subsequent  neighbouring  o f 2000 p o i n t s  substantially  from  were  To  80  10 m  0.5 cover  randomly  minimize  t h e r e f o r e based  of a t l e a s t  x  quadrats)  after  was a p p r o x i m a t e l y  and a b o r d e r  0.5 m  25  o f 60  Relative  (in  changed  the data.  surveys  Each quadrat  quadrat  each  1500 p o i n t s p e r f i e l d .  survey  not  collected  p o i n t s (5 rows o f 5) w i t h i n e a c h  up t o 500 p o i n t s from  per f i e l d .  data  'permanent' q u a d r a t s ,  of  f o r an i n i t i a l  discarding  points  a  on  field on  1500  10 m from  each  was  avoided  33  as a b u f f e r zone a r o u n d Estimates  of  different  species  projection  percentage  at cover  percentage  of  will  100%  cover  the  because  same p o i n t on  (Hubbard  Lolium  as  perenne  one  by  known  the  two  to  of L o l i u m  spp.  The one  one  were  readily  i n the  were t h e r e f o r e  may  quick perenne  in  nature  together  inspection,  identified  quadrat  of  Lolium  lumped  total  species  to d i f f i c u l t y  hybridize  Upon c a r e f u l  were o n l y  i n t e r m s of  examined.  m u l t i f l o r u m and  species  (vertical)  s p e c i e s i n any  Due  1).  recording a l l  these  l e a v e s of more t h a n  Lolium  species.  the  by  (Fig.  downward  points  ground.  are  multi florum  occurrences  the  a  expressing  25  the  between  1968),  were o b t a i n e d  and  every  of e a c h p a s t u r e  in  ground c o v e r e d  because they  recorded  point  for  differentiation and  cover  encountered  each  percentage  exceed  the p e r i m e t e r  and  specimens  1977  of  field; a l l  recorded  as  Lolium  data,  reciprocal  .  To  analyze  averaging,  the  e i g e n v e c t o r method of  p r o g r a m w r i t t e n by algorithm  percentage  Dr.  outlined  Gary  in Orloci  cover  o r d i n a t i o n was Bradfield  a  employed  developed  using a  after  the  (1978).  Results  the  The  mean  total  1977,  1958  and  respectively. species and  in  The the  as means o v e r  notable. general compressa  With  1939  3 fields the  over  was  the  study  159%,  186%  r e p r e s e n t a t i o n s of  are  study  shown f o r e a c h  field  repens  age, cover show  Dactylis  survey  and  212%  abundant  in Figure  Some t r e n d s  glomerata  while Holcus an  period for  t h e most  p e r i o d i n F i g u r e 3.  i n percentage  Trifolium  cover  pastures  relative  increasing  decline and  percentage  increase  are  shows  lanatus in  2,  a  , Poa  cover.  34  FIGURE  2.  Total  species present month s a m p l i n g  percentage  cover  of  the  i n the three p a s t u r e communities  14 most over  abundant the  33-  period;  1977 f i e l d ;  • 1958 f i e l d ;  1939  field.  35 PERCENT COVER  a) A g r o p y r o n  i  1  i June  1  1  1  1  1  1  1  1  1  1  1  Sept  Mar  June  Sept  Mar  June  1979  1980  1981  r  1 Sept  i Mar  1982  repens  36 PERCENT COVER  20  d) F e s t u c a  rubra  e) H o l c u s  lanatus  15 i  10 A  5i  40 I  30  20 A 10  r  T  i  1  r  T  1  r  f ) Lolium  perenne  40 4  30 H  20  10 H  —I  1—  June  Sept  1979  i  1  Mar  June  1980  r~ Sept  -i Har  1  1  r  June  Sept  Mar  1981  1982  37  1979  1980  198l  1982  38 PERCENT COVER  i)  1  1  —I  —I  r—  June  Sept  1979  1  1 Mar  June  1980  1  -  —J— Sept  1  Plantago  1  — r ~ —1 Mar  June  1981  1  lanceolata  1  V Sept  'Mar  1982  39 PERCENT COVER m) T r i f o l i u m  pratense  10 A  r  T  r  T  r  T  70  n) T r i f o l i u m  repens  60 A  50 J  40  30  20 -I  10 A  T  10  r  i  n  r  o) O t h e r  1 June  Sept  Mar  June  Mar  1979  1980  1  r  Sept June  1981  Sept  species  1 Mar  1982  40  FIGURE 3. field  The 10 most a b u n d a n t  based  Standard  on  percentage  errors are indicated  collected  and  cross-hatching of  mean  these  5  cloned  cover  depending  1976).  Analysis  was  w h e r e v e r an F - t e s t i n d i c a t e d Ar  - Agropyron  repens  Dg  - Dactylis  Fr  - Festuca  HI  - Holcus  lanatus  Lp - L o l i u m  perenne  glomerata rubra  Pp - Phleum p r a t e n s e Pi  - Plantago  lanceolata  Pc  - Poa c o m p r e s s a  Ra  - Ranunculus a c r i s ,  To  - Taraxacum o f f i c i n a l e  Tp  - Trifolium  pratense  Tr  - Trifolium  repens  each  The  period.  5  percentage  species  cover  age a r e s i g n i f i c a n t a t  u s i n g a MIDAS p r o g r a m on  study  e x p e r i m e n t s a r e shown by  on f i e l d  based  in  the study  bars.  Differences in  P<0.05 from an a n a l y s i s o f v a r i a n c e Guire  over  by v e r t i c a l  for competition  and s t i p p l i n g . species  species present  log-transformed  unequal v a r i a n c e s  (Fox & data  i n t h e raw d a t a .  PERCENT COVER  40 A  1977  Dg  Pc  Lp  Tr  To  Ar  HI  Ra  1958  50  PASTURE  PI  Tp  PASTURE  40 30 20 10 0 Pc  Tr  Dg  Fr  HI  To  Ar  Lp  1939  Pc  Tr  HI  Lp  To  Ra  Dg  Pp  Pp  Ra  PASTURE  Ar  Fr  42  Festuca  rubra  pasture  has  compared  to i t s cover  These o v e r a l l are 9  summarized sampling  species  the highest cover  that of  based  on  t h e 60 q u a d r a t s  total  i n each  percentage  field  d i s t i n g u i s h e d along  f o r 42% o f t h e v a r i a t i o n  t h e q u a n t i t y of v a r i a t i o n  (Fig.  the a x i s  i n the data.  accounted  2 accounts  For  axes  for  1  decreases  equality  lines  direction course  in  line  1 plane  which  The r e s u l t s  show  3 axes  f i e l d age.  accounted  with  increasing  pasture  successive a l l three for  the  axes  fields  1977  events  field  is  the p o s i t i o n i n g  axis  1 plot  an  F-test  f o r the  f o r the three pastures  along  In c o n t r a s t , t h e c o u r s e  axis 3 versus  age;  1 and 3.  sampling  on the  The t h r e e  and 3 however, t h e q u a n t i t y o f v a r i a t i o n  towards  1.  f o r each  pastures.  proceeding axis  4).  x  i n a l l three  a t P<0.05 f o r b o t h  showing  cover  increasing  for appreciable variation  of v a r i a n c e s o f a x i s s c o r e s  significant  (3 p a s t u r e s  f o r by t h e f i r s t  the o r d i n a t i o n g e n e r a l l y d e c l i n e s with  Axis  a  pastures.  i n an o r d i n a t i o n o f t h e 27 s u r v e y s  pastures are readily accounts  i n the other  (1958)  v e g e t a t i o n a l d i f f e r e n c e s among t h e 3 p a s t u r e s  events)  over  i n the middle-aged  Time-series follow  the  most  appears  same The  directional  o f t h e 1958 p a s t u r e line  course  the a x i s 2 plane. the  was  surveys  f o r t h e 1939 p a s t u r e on  a s an ' o s c i l l a t i o n  around  mean'.  B) COLLECTION AND PROPAGATION OF CLONES In  order  to investigate  several pairs  of n a t u r a l l y  (growing  in  physical  collected  from  locations  fine-scale  neighbouring  contact  competitive 'genets'  above g r o u n d  i n a l l 3 pastures  relations,  (or  clones)  i n the f i e l d )  where t h e r e  was  were the  43  FIGURE 4. axis drawn  1.  Ordination results b)  for  (circled  axis 3 vs. each  pasture  p o i n t ) t o the 1 977  axis  field;  of 1.  the  surveys.  Time-series  showing  1958  a) a x i s 2 v s .  course  lines  the p r o g r e s s i o n from  n i n t h sampling .  27  the  are first  event;  field;  1939  field.  44  1939 1977  /  \ \  \  1958 * \  /  » \ \  6  AXIS 1  AXIS 1  +  ( 4 1 . 7%  <»f t o t d l  variance)  45  greatest  overlap  frequencies collected species  i n the in  glomerata,  X/Y, its  repens  survey  lanatus  (June,  pairs  a l l three  (Fig.  X  respective  3).  genet  perenne  Each genet  was p r o p a g a t e d  ramets genet  of the o r i g i n a l  provided type.  a  stolon  or  with  stock  a 'type'  name  an  approximately  one t r i f o l i a t e  Each  leaf  of  such  of c l o n e d  setups,  X/Y  and  The p a i r s were and e a c h  conditions  stolon  (physiologically  clone.  from a g r a s s  Hence,  glasshouse  or c u t t i n g  supply  In e x p e r i m e n t a l  8-10 cm t i l l e r  removed,  Dactylis  root material)  s e p a r a t e l y under  of t i l l e r s  Y)  genet p a i r .  (including  by r e p l a n t i n g a s 'ramets'  individuals)  abundant  , Poa c o m p r e s s a a n d  was g i v e n  (and X d o e s n o t e q u a l  periodic separation  followed  were  were  i s i t s s p e c i e s name a n d Y i s t h e s p e c i e s name o f  a s two whole p l a n t s  type  they  cover  Clones  5 o f t h e most  pastures;  , Lolium  percentage  1979).  for  would be a n a t u r a l n e i g h b o u r i n g  collected  one  to  n a t u r a l neighbour  Y/X  by  first  Holcus  where  their  a l l possible  common  Trifolium  in  pieces  independent  population  individuals  of the  a 'ramet' was d e s i g n a t e d  clone 2.5  with  a l l root  of  as  material  cm s e c t i o n o f w h i t e c l o v e r  and an a s s o c i a t e d a x i l l a r y b u d .  SOIL SAMPLING AND ANALYSES Soil following nitrogen,  samples  from a l l t h r e e  variables: total  carbon,  exchangable calcium,  pH  fields  were  ( i n water), total  cation  total  analyzed  phosphorus,  exchange  magnesium, p o t a s s i u m  f o r the  capacity,  and sodium.  total and  46  Methods A  reciprocal  averaging  ordination  s p e c i e s c o v e r ) from t h e March survey  before  fields. stands  t h e range  20  stands  ordinations  so p r o d u c e d  ordinations vegetation  shown  At  edaphic  each of the  samples  were  Chapman e t a_l . by  2  of these o r d i n a t i o n s ,  were s u b j e c t i v e l y  present  of  9 used  to test  a given  field  selected  i n May  sites  for  14 d a y s ,  quadrat. the  12 s o i l  preparation All  procedure  composite  site.  outlined using by  determination.  soil  cores,  each  not  to  which  15  cm  the outside disturb  the  samples  were  gave  a  mean  the  entire  soil  sampling  composite  sample  was a i r d r i e d  s i e v e and t h o r o u g h l y mixed i n  analyses.  in  t h e ammonium  out  on  Lavkulich fluoride  auto-analyzer Total  field,  12 i n d e p e n d e n t  of  t h r o u g h a 2 mm  for soil  each  from a r o u n d  sample  Each  a n a l y s e s were c a r r i e d  procedures extracted  These  representative  sifted  c a n be a c c o u n t e d f o r  1981 by t h e c o m p o s i t e method ( e . g .  of  volume a t t h e q u a d r a t  environmental  the h y p o t h e s i s that  in  interior  value  the  The  variation.  o f t h e p o s i t i o n e d q u a d r a t so a s  analytical  field.  the b a s i s  cm d i a m e t e r , were e x t r a c t e d  into  each  formed  perimeter  amalgamated  within  20  chosen t o  to re-ordination.  1940) a s f o l l o w s :  the  out f o r a l l three  subjected  within  20  taken  recent  then  in Figure  differences  underlying  fields  ( b a s e d on  ( t h e most  carried  1 plot  of v a r i a t i o n were  survey  was  2 vs a x i s  from e a c h o f t h e t h r e e  These  deep  sampling)  From t h e a x i s  represent  by  soil  (1981 )  of quadrats  was  carbon a n a l y s i s  the  2  mm  (1978). method. used  for  fraction  Phosphorus A  by was  colourimetric total  nitrogen  was o b t a i n e d u s i n g a  'Leco  47  FIGURE  5.  selected Total cations  Mean  stands  cation  values  from  each p a s t u r e  exchange  are expressed  for chemical  capacity  as meq  per  analyses  of  (4, 23  and  42  (CEC)  and  all  100  g  soil.  soil years  from  20  old).  exchangeable  48 38.D-, 36, OH O  5.60H x x a c •H  w  34.0  5.50  32.0  5.40'  30.0 4.6  05  28.0  4.4  26.0  4.2 + +  24.0 a a  O  X  to  n  4.0  22.0  3.8  20.0  3.6  J8v0  3.4  0.43  1.10  Oh  • J 2; <  U3  H O O O H OS H Vi H 2;  0.42  +  +  0.41  S  1.00 0.90  0.40  0.80 j  5.80  0.25 -,  5.60  %'  0.20  2  O CO  5.40  0.15  5.20  0.08  < u I—I  < H O  5.00  0.07 +  CO  4.80-  55  4.60-  0.06 0.05  23  42  1 •4-  YEARS SINCE PLANTING  49  FIGURE 6.  Distributional  the  2  axis  selected cations  versus  quadrats are  i n meq  from per  p a t t e r n s of  axis each 100  1 plane  of  pasture. g of  soil.  the  9  soil  the  stand o r d i n a t i o n  Values  for  variables of  on 20  exchangable  50  •5.25 • 5.20  PH  • 21.1 • 12.1  P  1977  1977 •^27.1  •5.65  . 15.1  * 5.60  • 5.'l5  •19.2 •27.1  5.65 •16.1  • 15.1  •5.70  *5.55  •19.2  *5.50 •5.60 •5.60  ,  • 11.3 • 11.5  •26.2  *5.50  •20.8  5.20  •5.50 1.50  ,  16.9  •18,9 21.6 •5.10 5*.50  • 5.10  •21.7  -21.7  5.20  13.5.  •11.5  •5.50  P  pH  1958  •5.30  1958  • 12.2 •15.1  •5.50  • 5,30 5.60 •  23.E •  5>75  2  y-  5.50« • 12,0  • 5.60 • 5.65  •5.15 5  • 5.80  w  515 ^ ' , , "5.55 5.80 5.60  13.2  . . * 23.9 16.1 25.8  • 25,5  • 5.60 26,1  • 5,60  • 5,70  > 21.6  ig.e  51  '0.416  • 5,44 • 5.77  C  * 0.463  N  1977  1977  •0.425  •5.92  •0.259  • 3.39  •0.431  •5,07 •0.278  •4,43  • 0.278 • 0.469  •0.366  •5.11  •3.94 •6.86 •6.60 • 4.99  •6.09 •5.39  .0.525 • 0.466 0*469  ,  6.71  ,•5.80 5.97 •E.16 . * £,91  *0.53 0.463 0.522  e  „ E.75  •0.425  • 5.58  N  c  1958  • 0.431  1958  • 6.42  • 0.372  0,481  7,66 0.391 •  0.422  6,12 -  0.491-  • 0.447 0.372  0.431.°'^ * \ . •0.4280.419 0.366 0.338  6,44 • 5.3D  . . * 6-39 6,03 5.03 4.56 • 5.69  • 0.384  •0.366  0.328  • 5.65  •0.494  •0.347 •0.388  N 1939 •0.375  • 0.369 •0.369  • 0.403  •0.469  • 0.341  0.403  .0.400  0.413 •0.369  •0.366 • 0.422  ^  0.350  4.79 7.08 -  5.95  52  •37.6 •35.8  CEC  •3.0  Ca  1977  ++  1977  •37.6  •5.7  •31.3  • 3.6  -3M.0  •3.9  •31.0  •1.2  • 31.0  •36.7  •3.1  * m.t  *3.6  •6.1  •10.3 •31.9  • 37.6 • 32.2 ,  •59  * 5  9  39.1  ,• 12.1 11.2  /1.2 1.7 • '" •15.6  -5.9 • 5.1  -10.3  5 9  ?  12.;  • 3.6  • 30.1  Ca  CEC 1958  32.2  ++  1958  • 2.1 • 3.1  > 31.3  •3.6  • 32.2 35.8 •  1.9 •  37.6  • 36.7 •30,1  .  a  • 2.5  37,61 5.8 . . * 35.7 ia.2 10.3 33.1 i  3,6 6,1 •  11.2 • • 1.6  9  5.1  1.9  6  '1  . . "6.9 5.1 1.1 3.2  • 1.2  • 31.0  •1,0  • 3.9  . 36,7  • 30.1  • 3.5  •32.2  •3.7  • 26.9 •31.3  CEC  Ca  1939  •1.0 •3.9  ++  1939 •2.9  •30.1  •31.3 26.0  •26.9  • 36.7  • 21.2  •1.9  3Q.1 •26.0  •3.6  •1.2  •1.7  •29.5  3.7 3.6  •1.0  32.2  •2.9 •2.5  •26.0  •1.7  •32:2  •1.1  • 38.5  y, Q•27.8  *33.1 •32.2  •3.7  •3.0 •3.6 '  M  3.7  53  Mg  +  •0.49 •0.43  +  K  1977  • 0 19 •0.17  +  1977  •1.26  • 0.19  .0.95  • 0.17 •0.62  •0.2C •0.84  •0.62  0.13  • 0.82 • 0.97  •0.20 •1.11 •1.01  • 1.03 • 0.97  .  • 0.17 • 0.17 • 0.06  0.99  ,•0.70 0.97  • 0.19  •0.13 •0.83 •0.26  / 0.06 0*.26 • 0.19  • o.oe  *0.82  • 0,13  0.88  • 0.64  0.06  • 0.16 K  1958  • 0.37  +  1958  •0.88  > 0,16  •0,72  • 0.06 2.18-  0.13 -  l;*  H.19  0,70 • • 1.42  1.4E 0.86 •- ~ • . . * I.* 1.50 0.80 0.66  • 1.85  • 0.99  „ „ •2.06  • 0.51  •0,70 Mg  +  •1.07 •0.80  +  1939 •1.15  •l.U 0.76  0,86  > 0.86  • 0,41  • 1.03  •0.64  •0.41 • 0.76 •1.30 •0.58  •1.69  .1.07  *°'  • 0.20  • 1.07  0.37  • 0,19  3,84  a.si  -CIO °-' . , * 0.1E 0.17 0.16  1 9  X  -0,17  54  Na  •0.O5 •0.01  +  1977 •0.08  •0.11  •0.05 •0.10 • 0.08 • 0.09  •0.03  •0.08 •0.13  • 0.17 • 0.01 ,  0.09  /0.05 • 0.09 * • 0.10 • 0.01 0  r o  o.qp  • 0.07  Na  +  1958  • 0.03 •0.07  • 0.05 0.08 •  P.-09 0.15 •  • 0.05  0.11 -« * ° ^ . ' 0 . 1 2 o.lO 0.08 0.01 0  • 0.09  •0.05  .„  08  • 0.05  • 0.23  •0.08 Na  .o.io  1939 •o.oi  •0.01 . nu ,(lKc  OJ33 *°-  • 0.08  •0.01  Q 2  • 0.01  • 0.02 •0.03  •OvOl  • 0.08 •0.09  „ -0.15 •0,03 m  •0.07  O.Oi  55  analyzer'.  The  cations  total  and  ammonium a c e t a t e cation  method was  used  exchange c a p a c i t y  for  exchangable  (CEC).  Results The  mean r e s u l t s a r e  significant between  differences  fields  collected  was  of  for  field.  1 plane  of  the  the  previously. variables axis  quadrats are  given  measured  6  soil  of  the  values as  the  20  that  to  of  7.  A  the  few  the the  be  soil  stand axis  of  variables were  not  ordinations  2 versus  quadrats the  axis  mentioned  different  species  and  significant  soil  with  above o r d i n a t i o n  gross vegetation  some e x t e n t  test  the d i s t r i b u t i o n a l  for 10  A  samples  selected  3 a x e s of  in Table  may  on  coefficients  first  suggesting  soil  i s b a s e d on  5.  for  illustrates  percentage cover  shown  in Figure  variables  Each diagram  f o r the  field  edaphic  Figure  Correlation  scores  mean  possible  ordination  with  were f o u n d  in  not  randomly.  patterns each  summarized  the of  20  correlations  differences  within  a  accounted  f o r by  underlying  of  series  of  variation.  DISCUSSION The  u n i q u e management  different  aged  coexistent  s t a g e s of  natural  pastures  sequence  time-series planting  (i.e.  provides  community  of  survey  history  results  age)  is  conditions  It (e.g.  must  be  approximation  evolution  development. for  This  grazing  . three  of d i f f e r e n t to  the  i s corroborated cover.  therefore  vegetation  recognized  climate,  belonging  percentage  assummed  v a r i a b l e d i s t i n g u i s h i n g the pastures.  an  this  to  structure however  that  . intensity)  by  the  Time  of  be  of  same  the  a  major three  environmental at  particular  56  TABLE 7.  Product  measured  soil  variables  t h e most abundant axis  scores  selected  moment  for  quadrats  c)  1939 p a s t u r e .  at  t h e 1% l e v e l ) .  correlation with quadrat  species overall the  first  3  (Fig.  6).  a)  (* s i g n i f i c a n t  coefficients percentage  at the axes  of  for  the  9  cover  f o r 10 o f  site  and  with  the o r d i n a t i o n  of 20  study  1977 p a s t u r e ;  b)  a t t h e 5% l e v e l ;  1958 **  pasture;  significant  a)  1977  PASTURE  pH  Agropyron  repens  • 345  P  -.179  D a c t y 1 i s glome rata  .059  .502*  Festuca  .1*55*  .185  rubra  N  C  -.460*  -.451*  .212  .266  CEC  Ca  + +  Mg  + +  K  +  Na  +  -.261  -.251  -.048  -.072  .128  .170  .364  .276  .361  .276  -.642**  -.524*  -.345  -.207  -.061  -.087  .088  -.505*  -.085  -.368  lanatus  -.2*40  -.258  -.109  -.107  -.149  -.406  L o l i u m perenne  .212  .138  -.009  .075  .305  .193  .070  .064  -.036  Phleum p r a t e n s e  .244  .342  -.105  -.147  -.267  .092  .244  -.016  -.059  Holcus  -.362  -.474*  .295  .164  -.069  .030  .225  .173  .137  T r i f o l i u m repens  -.299  -.221  .325  .379  .451  .216  .111  -.187  .130  Ranunculus  -.076  -.333  .025  .027  .001  .026  .084  .371  .268  -.411  -.376  -.159  -.062  -.202  -.224  -.071  Axis 1  -.505"  .022  .320  .185  -.026  .073  -.108  Axis 2  -.048  .042  -.374  -.421  -.477  -.506*  -.419  .029  -.270  Axis 3  .216  .382  .127  .074  -.125  • 359  .075  .109  Poa  compressa  acris  Taraxacum o f f i c i n a l e  .512*  .503*  .507-  .490*  .130  b)  1958 PASTURE  pH  P  N  C  CEC  .063  .120  -.196  -.020  .141  .044  .483*  .261  .261  .086  -.142  .310  .298  rubra  .372  -.043  -.388  -.092  -.201  lanatus  -.468*  -.615**  .172  .059  -.530* -.167  Agropyron Dactyl is Festuca Holcus  repens glomerata  Ca  + +  Mg  + +  K  +  Na  + +  -.099  -.001  .341  .342  .164  -.365  -.031  -.096  .214  -.565**  -.295  -.206  -.406  .336  -.334  • 385  -.068  .547*  L o l i urn perenne  -.149  .395  .138  .029  Phleum  -.048  .117  .577**  .585**  .513*  .661**  .161  .489*  .251  .201  .506*  .367  .347  .003  -.047  .356  .478*  -.160  -.221  .349  .646**  .387  .199  .114  -.251  -.404  -.244  Poa  pratense  compressa  T r i f o l i urn repens Ranunculus  .361  -.609*-  .298  .210  .361  -.342  .306  .030  -.163  -.285  .292  .232  .010  .201  -.180  Axis 1  .280  .526*  .115  -.012  .557*  .702**  .340  .283  .130  Axis 2  -.540*  -.419  .458  .166  -.218  -.201  -.185  -.002  -.414  Axi s 3  -.234  .174  -.264  -.266  -.440  -.004  -.172  -.242  -.035  Taraxacum  acris  -.097  officinale  -.529*  c)  1939 PASTURE  P  N  C  .016  -.134  -.1 10  .037  .117  .100  -.178  .376  .500*  -.132  -.390  -.303  -.035  -.128  .097  -.308  .050  rubra  .051  -.222  .105  .083  .438  .135  -.112  -.030  .100  lanatus  -.562**  -.013  .296  .230  .249  -.079  .083  .146  .199  • 307  -.334  -.141  -.016  .224  .078  .034  .165  -.056  -.194  .407  .156  .159  -.110  .038  -.133  -.015  .329  .036  -.023  .089  -.242  .203  -.050  .012  -.173  .453*  .368  .032  .017  .170  .193  -.111  -.045  .031  -.336  .080  .221  .197  -.092  .159  .031  .149  .260  .214  -.349  -.172  -.146  .043  . 142  .245  -.060  -.128  Axis 1  .569**  -.147  -.167  -.029  .042  .278  .034  -.056  -.024  Axis 2  .404  .312  -.210  -.195  -.231  -.042  -.070  -.228  -.183  Axis 3  .096  -.197  .045  .189  -.243  .270  -.017  .092  .054  PH  Agropyron Dactyl Is Festuca Holcus  repens glomerata  Loliurn perenne Phleum Poa  pratense  compressa  T r i f o l i urn repens Ranunculus Taraxacum  acris officinale  .346  CEC  • 113  Ca  Mg  + +  K  +  Na  +  60  times as  (e.g.  the  mixture  relative (Table  fields. over  during  6) a r e l i k e l y  importantly,  the  substantially  among  species  of  relative  abundant  species  repens,  i n t h e 1977  that  show a d e c l i n i n g t r e n d  field  age.  species  (now  fields  the  present  i n the  i s at  least  abundance,  second  and  differs  3).  Some  and  others  age.  fourth  a r e t h e two most species  pasture  43  years  there  pasture)  have  that  have  Poa most  dominant  (with  the  multiflorum  e x t i n c t i o n with  after  More  species  and p e r h a p s L o l i u m  1977  field  (Fig.  T h e r e a r e no  to t o t a l  1939  each  increasing field  the  i m p l i c a t i o n i s that of  three  patterns.  various  pasture,  of T r i f o l i u m p r a t e n s e  establishment  of  with  i n both older p a s t u r e s .  The  growth  aged  abundance  i n the  not s u r p r i s i n g l y ,  seasonal  well  i n the p l a n t i n g  changes w i t h i n  of i n c r e a s i n g r e l a t i v e  and T r i f o l i u m  exception  species  been d i f f e r e n t  species  different  compressa  species  the  representation  show t r e n d s  decreasing  of  p e r i o d and t h i s ,  reflective  f o l l o w i n g p l a n t i n g ) , as  t o have  of v a r i o u s  study  partially  immediately  proportions  The c o v e r  the  and  )  increasing since  been  no  been  the major  totally  eliminated. Trends a l s o occur field  age  (Fig.  consequence  of  Within-community variables The  oldest  significant community for  by  in several s o i l  5) w h i c h may (or  v a r i a b l e s with  be e i t h e r r e s p o n s i b l e ' f o r ,  neither),  the  trends  c o r r e l a t i o n s of s p e c i e s  however (1939)  was  not g r e a t e r  pasture  in  with  fact  c o r r e l a t i o n s of the t h r e e vegetational  underlying  patterns  edaphic  increasing  in species  cover  with  had fields  notably  the  (Table  in  the  the 7).  a  cover.  increasing pasture  are therefore  variation  or  soil age. fewest  Within-  l e a s t accounted oldest  pasture  61  compared w i t h t h e two younger The o r d i n a t i o n represent respect  a  sequence  to o v e r a l l  youngest  and  respect  there  vegetational  (Orians in  here  1975).  those  that  pasture  community  pastures is  patterns  community  was  of  indicate  more  i s used  composition  are  i n the sense pasture  over  differences;  their  three  study  cover  variables  are  large.  A  f o r each  survey  cover  of  each  differences period.  species  were  In  found  spite  vegetational  of  patterns  p a s t u r e s as d i s t i n c t its  peculiar  characterized community significant lack  of  variables  increasing  constancy  the time  'community  fields  however  clear  this  toward  are v i r t u a l l y  identical  show  differences  based  on  in  soil  index  (Peet  total  percentage  ( T a b l e 8) and no  significant  data  exposes each  here the  on three  distinguished  developmental  community  constancy.  is  accompanied  not  the study  presented  (Fig.4)  common  trends with  on t h e q u a d r a t  p e r i o d and  striking  and  change i n s p e c i e s d i v e r s i t y .  any  variability  no  entities',  a  4).  'constancy'  t h e most study  the  (Fig.  the t h r e e p a s t u r e s over  this.,  in  in  community  showed  in this  time  Simpson's d i v e r s i t y  per f i e l d  over  placement by  varies)  amongst  the  ( F i g . 4).  relative  calculated  particular,  trend i n the o r d i n a t i o n  (although  1974) was  of  species compositions  not  with  p a s t u r e age  t h e 33-month  in older pastures the  over  fields  development  t h e most d i f f e r e n t  increasing  r e p r e s e n t e d by a d i r e c t i o n a l  Superficially  In  stability  with  The y o u n g e s t  quadrats  the three study  vegetational patterns.  oldest  and  'Stability'  results  pastures.  by  series  Increasing by  any  T h i s together with the  increasing  p a s t u r e age f o r s o i l  o r d i n a t i o n s ( F i g . 6, T a b l e  7)  points  62  TABLE  8.  Simpson's  survey  based  on  total S  field.  diversity  indexes  percentage  (D) c a l c u l a t e d  cover  of  each  f o r each  species  per  2  (D=1-  of  E (p.) , where p. i s t h e p r o p o r t i o n of i n d i v i d u a l s i=i s p e c i e s i i n t h e community and S i s t h e number of s p e c i e s ) .  The  mean v a l u e s o v e r  1  different analysis An  amongst  the the  study  period  were  three  fields  (P=0.3697)  of v a r i a n c e u s i n g a MIDAS p r o g r a m  F-test  analysis  1  not  (Fox  f o r t h e h o m o g e n e i t y of v a r i a n c e s was  was  performed  SURVEY  SURVEY  NO.  DATE  on  untransformed  data.  PASTURE 1977  1958  1939  1  June  1 979  .859  .863  .822  2  Sept.  1 979  .809  .862  .832  3  Mar.  1980  .805  .790  .755  4  June  1980  .829  .845  .845  5  Sept.  1 980  .859  .842  .864  6.  Ma r .  1981 .  .854  .831.  .824  7.  June  1981  .854  .868  .842  8  Sept.  1 981  .870  .888  .852  9  Mar.  1 982  .789  .844  .818  .836  .848  .828  Mean  significantly based  & Guire not  on  an  1976).  rejected  and  63  to  the  question  grazing  and  properties  of  the  neighbour and  importance  of  interactions)  development  of  biotic in  neighbour  factors  (e.g.  determining  the  relationships in  these  pastures. It  i s important  have been e x p o s e d since  last  e.g.  be  to  a f f e c t e d by  is  also  may  be  older  in  expect how  and  the  had  hence  a  attention  on  scale  of  events occur  at  level  forces  involve  the  concerning  follow.  the  r e l a t e d to  (e.g.  competition) may  not  be  time  in  grazing  may  in this  chapter  of  biotic selective  Such  occurring  micro-scale  addressed The to  at  pursued  assumes however  selective  in  that  neighbour and  the  critical  constitution.  simultaneously  of  accommodation.  response  genetic  It  components  succession.  separable.  dynamics  interactions  community.  and  is  operating.  biological  or  It  f o r subsequent  i n t e r a c t i o n s are  operate  readily  been  adequately  discussion  both  and  history  individual  changes  Subsequent  structure  longer  vegetational  selection  (Kemp 1937)  ancestral  the  (i.e.  is preferential).  d y n a m i c s of  not  of  important  the  generate  neighbour  pressure  effects  a  pastures  grazing  growth h a b i t  that  longer  and  traditional  to  t i m e of an  three  findings presented  fact  within-community, e v e n t s are  issues  of  s e l e c t i o n has  neighbourhoods within  and  the  provide  community  the  have  to accumulate  local  may  such  that  a t t r i b u t a b l e to  focuses  that  ( i f grazing  that  long  reasonable  interaction  This  abundance  communities  forces  Grazing  in a f f e c t i n g plant  relative  reasonable  i n mind  to d i f f e r e n t lengths  planted).  pressure, species  to bear  These  chapters selection  interactions that  their  64  CHAPTER 3  A QUALITATIVE MODEL FOR PASTURE PRELIMINARY  COMMUNITY EVOLUTION:  EVIDENCE FOR SPECIES INTERACTIONS AND COEXISTENCE USING CONTACT  SAMPLING  65  INTRODUCTION Species  interact  in nature.  is  often readily  Yet  between p l a n t s , s p e c i e s  immediately  observable,  obvious.  and  e c o l o g i s t s even  of  empirically  in  area  and  provided Harper Bazzaz to  Fowler  given  to  climatic)  populations  to  population  common  the in  (e.g.  the  the  this  tacit  of  those  (Sagar  also  & Harper  & Forman  important  Wiens  however,  1977,  most  yet  remain  Murdock  as  interaction  on  arises  only  really  & B i r c h 1954;  see  conditions,  undisturbed 1961,  1976).  &  attention  disturbances  factors  controlling  Caswell  1978,  Menge  an  innate  have  i s necessary fairly  interaction attempts  -Darwinwill  an  be  virtually  to g e n e r a l i z e  t o view  determined  for  constant  in  a l s o H a i r s t o n e t ajL.  than  have  Abul-Fatih &  of M a l t h u s and  with  an  Putwain  resource, l i m i t a t i o n  intensity rather  from  other  than  I t i s more r e a l i s t i c  of v a r y i n g  limiting  f o r more  average  competitive  not  competition  ( o r u n i m p o r t a n c e ) of c o m p e t i t i o n  1966).  (Pianka  that  often  impossibility  1976,  species  many more o f f s p r i n g  and  importance  left  this  truism.  s p e c i e s removals  of p r e d a t i o n and  1975,  and  are  Some have c a l l e d  assumption  Dispute  Andrewartha  phenomenon  on  :  nature  environmental  centered  s i z e . . T h i s echoed paradigm  versus an  is  resources  Allen  role  which are  replacement  inevitable. about  1981).  produce  population  supports  1975,  (Connell  Against  capacity  in  Pinder  perhaps a  in nature  has  s t u d i e s on  response  ecologist  i n the v i r t u a l  that  some q u a n t i t a t i v e d a t a  1979,  1979).  enigma  demonstrating  subsequent  1970,  be  (e.g.  the  animal  u n p r o f o u n d , and  interest  f a c e an  Only  the  interaction  Most  some c o m m u n i t i e s .  To  nature 1960  competition by  prevailing  all-or-nothing  66  A  persistent  interaction  they are c o e x i s t i n g . into  the  notion  arbitrary; place  of  for mobile organisms.  s p e c i e s must  Mobile  relations  with  component  for sessile  inherent  question exists  an In  interaction  in  physical  local  found  up t o 69%  Sampling not  based  the  of  individual  which  reflect  relevant  p e r s p e c t i v e may  v e g e t a t i o n (Harper on p h y s i c a l A plotless  which  variation  enables  scale,  from  contacts.  I t was modified  the  by  spatial  elicits  individuals. interactions  plants any  often in means  in  plant  dune  annuals  and  c a n be  a r a d i u s of 2 cm. i n quadrats  may  and a more  "plant's-eye-view"  of  1979a)  individuals. v e g e t a t i o n has been  of  community  designed  Turkington  of  (1977)  weight  of. i n t e r a c t i o n s ,  "plants-eye-view"  originally  their  1977b, T u r k i n g t o n & H a r p e r  of sampling  investigations  sessile  Mack & H a r p e r  sand  be t o t a k e a  1977a,  component  The  has  present within  c o n t a c t s between  method  variation  further  involving  records s p e c i e s presence/absence the  same  altering  displaced  rarely  of  the  of p r o x i m i t y  neighbours.  f o r by t h e n e i g h b o u r s  appropriate the  output  to  built  i s more e q u i v o c a l - t h e  degree  neighbourhoods  that  somewhat  for  s u c h as p a s t u r e s , s p e c i e s  i t s proximity  reproductive  in  that  while  however  c o n t a c t and t h e i n d i v i d u a l  that  be  another.  between two p e r m a n e n t l y  very  seem  The s p a t i a l  from  one  a s t o what  adjusting  accounted  to  organisms  some v e g e t a t i o n t y p e s  occur  .  interact  respect  components a r e  i n some way  i s very d i f f e r e n t organisms  s p e c i e s means  both of which  f o r some l e n g t h of t i m e  organisms  two  and t e m p o r a l  coexistence,  coexisting  together  spatial  Spatial  between  by  (1975)  in  structure  terms  Yarranton and  developed  used  of  and  neighbour  (1966)  and  recently in  67  grasslands 1977,  as  the  Turkington  This  sampling  coexistence reference fixed  'contact & Harper  sampling 1979a,  scheme d e f i n e s a in  plants.  It  because neighbours  individuals  which  organization, i.e.  randomness smallest  of  inherent  methods of  in determining  particular  species  intervals  (Stowe  analysis"  of F o w l e r  methods  - the  also  &  neighbours  along  a  and  the  1979)  & Antonovics these  problems size  and  grass-legume  communities,  predominantly association grasses in  a  and  negative amongst  (1981)  transect  legumes.  50-year-old  Trifolium  repens  and  permanently p o s i t i v e  seasons,  the  are  The  pasture,  all  e_t  of  Harper the  distinct in  their  method  grasses,  in  found  negative  association  between  found  that  a s s o c i a t i o n s between  e a c h of e i g h t m a j o r g r a s s e s or p e r m a n e n t l y  study,  (1977)  (1979b)  point  plotless  two  a_l.  positive  and  small  latter  sampling  amongst  and  method  at  similar  revealed  contact  Turkington  Turkington  the  scale  "neighbouring  advantages.  association legumes  at  shape f o r a  a s s o c i a t e d g r a s s l a n d s p e c i e s which d i f f e r e d Using  non-  of  groups  of g r o w t h .  that a  detected  i n two  seasonality  of  hence,  c a r r i e d , out of  different  concept  "species-juxtaposition"  sequence  offering  ground.  neighbourhood.  a p p r o p r i a t e quadrat  Wade  laterally  t o t h e way  is  for  frame of  above  and  a v o i d the  The  those  contact  i t is sensitive  sampling  component  neighbourhood  local  1979).  unequivocal as  association  vegetation type.  recording  a  immediate  interspecific  possible scale  Plotless  of  its  an  regarded  embodies  et a l .  spatial  physically  community  experiences  precise  are  the  ( T u r k i n g t o n e_t a_l .  1979b, A a r s s e n  provides  Moreover,  plant  method  method'  negative  remained  over  a  either  16-month  68  period. Contact technique of  of  for providing  species  using  sampling  coexistence  the  contact  quantitative  data  i n communities.  s a m p l i n g method,  seasonal  and t e m p o r a r y .  types of a s s o c i a t i o n s one w h i c h  is  association evidence sphere  identified  that  influence  negative  evidence  sense)  of  interaction. there  present  to provide picture  of  Antonovics  et  the  some  nature  drew  (1979)  and  on t h r e e  age  last  (since  respectively.  and  is  similar  association  stable  positive  i n t e r p r e t e d as  within  each  other's  by c h a n c e ) .  (in a  A  interpreted  non-cognizant  i s not evidence individuals  against  that  enter  sampling  method  for constructing dynamics  site  of  attention  Saulei  occassions  By c o m p a r i s o n ,  in pastures  influence.  basis  ploughed)  categories  hand c a n be  however  t h e p r o c e s s o f community  a l .  three  i n v e s t i g a t i o n uses the c o n t a c t  i n the study (1978)  This  (1979),  than expected  on t h e o t h e r  sphere of  associations  three  A  sampling  often  et a l .  A stable  time.  frequently  may a l w a y s be  a quantitative  interactions  quantify  (more  association  one a n o t h e r ' s The  exist  measure  (1981) r e p o r t e d  f o r some measure o f a v o i d a n c e  coexistence; into  two s p e c i e s  contact  devised  a  association  fields.  through  by  reflecting  Aarssen  Saulei  i n abandoned  constant  detected  of  stable as  t h a n any p r e v i o u s l y  s i g n i f i c a n t (P<0.05) i n t e r s p e c i f i c  stable,  is  goes f u r t h e r  by  of  change  aged  the lack in  (1981)  studied  range  the present  species  pastures.  of attempts to  pastures.  i n p a s t u r e s which a  preliminary  fine-scale  different to  a  of  interspecific differed  in  and  8 years  investigation  concerns  p a s t u r e s o f w h i c h no two a r e l e s s t h a n  18  Aarssen  20 y e a r s  different  69  in  their  ages,  encompassing question change  in  of in  central species The  'stable' younger  role  that as  the  pasture  (e.g.  (e.g.  Gray  Wilson  properties  community-level  1971).  Data  relationships  will  work r e p o r t e d  the-  between s p e c i e s  1980),  of  neighbour  way(s)  in  or  which  f o r the  association  of  'Community  in  assemblages  the p a t t e r n s  sense  be  viewed  of  selection fine-scale  f o u n d a t i o n f o r the to . i d e n t i f y  competitive  i n response to neighbour  may  group  the  natural  designed  in  paleoecolgical  rather  also provide a  later.,  more  'temporary'.  i s proposed  Darwinian on  have  or c o m m u n i t y - l e v e l  but  are  p a s t u r e s than  pastures  sense  1981)  products  & Woodwell  precisely  e t a_l .  community  evolution.  the  The  t e s t e d : There  interspecific  in  1976,  be  a model  community not  years.  pasture  short-lived  of  3  in older  younger  of m u l t i - s p e c i e s  (Whittaker  experimental  during  of t h e r e s u l t s , patterns  of  of study  What a r e t h e p a t t e r n s o f  hypothesis w i l l  whereas  i s used here  progression  is:  are r e l a t i v e l y  basis  characterizing  selection  term p e r i o d  (unchanging) a s s o c i a t i o n s  changing  evolution'  longer  associations  following  which  the  of  a  importance  pastures,  associations On  involves  surveys i n 3 seasons over each  development? more  and  relations  more change  interactions.  METHODS  Vegetation Data 0.5 each  Sampling were c o l l e c t e d  from  60  systematically  m q u a d r a t s i n e a c h of t h e t h r e e quadrat,  contact-samples  fields were  a r r a n g e d 0.5  (Fig.  taken  at  m x  1).  Within  each  of  25  70  systematically samples  per  collection  field.  at  recommended sampling  from  of  is  the  of  a  and  point  that  species.  In  point  closest  to  no  sample  point  advantage grassland of  former c a s e ,  mate'). species  species  original species  and  species  of  contact  with  I f the  contact  (i.e.  j=i)  mate of or  no  resampling  j  a different it  is  with  the  contact  is ultimately  interspecific i s no  a  mate  then  until  point  i  removed  interspecific  sampling has  sample  i s in contact  the  where t h e r e  rejected  of  i s gathered  this  i s that  vegetation contact,  of  the  featuring  there  d i f f e r e n t parts Each  the  the  strategy;  neighbourhoods  two  point  each  downward  subsequently  i s determined,  different  or  is  the  At  vertical  initial  data  follows  (in press).  a contact  no  whatever,  information  sampling  point  species  point  is  until  contact-  point  contacts  species,  occurs  for  the  another  species. No  this  point  point  1500  here  the  nearest  t o the  the  Where the  contacts  entire  the  resampled  point  or  to  by  i s the  sampling  species  from the  chosen.  The  hit  pin  used  et a l .  i n d i v i d u a l j ('contact  rejected  established  Jong  i.e.  o p t i o n a l methods f o r  method  de  sampling  location  identical  species  The  of  points,  several  individual  species').  neighbouring is  each p o i n t .  point,  that  sampling  There are  strategy  projection ('point  arranged  the  above method on  of  the  intraspec1fic association  procedure  selectively  interspecific i t avoids  of  the  focuses  interaction.  often  formidable  the  two same  different  on  task  involved,  in the or  plant.  pasture same  plants  in  The  t r y i n g to d i s t i n g u i s h whether, at  are  three  on  c o m m u n i t i e s was  dates  as  the  surveyed  surveys  for  using the  71  percentage summer out  cover  ( J u n e ) and  starting  in  of  March  three  i n June  1982.  intermittent  species, fall 1979  Winter  i.e three  (September) and  s p r i n g (March), surveys  terminating with  surveys  were  not  were the  three  carried  last  possible  survey due  to  snowfall.  Data A n a l y s i s The  hypothesis  tested  was  random  species.  number  of c o n t a c t s between s p e c i e s does not The  p o i n t s p e c i e s and  ordered  contact  selected  p l a n t , and  the  pair  appropriate  displaying  the  this  of  amongst  expectation.  Randomness i n  that  layout  number  the  i i s the second.  of t h e d a t a  of  each  means  deviate  that from  i t s c o n t a c t mate  ( i , j ) where  j t h a t of  context  association  define  9  )  an  first  illustrates  i n a complete  pair  random  s p e c i e s of t h e Table  the  count  observed  matrix  after  N  samples. Given the m a t r i x where  along  cell  between in  the count  the d i a g o n a l  ( i , j) ,  species  i<j  i and  which the p a i r s  1966,  Turkington  contingency this  which  correct  et  al. such  De  data  de  analysis  in  a  has  number of  distinguishing Earlier  studies  Turkington  using  the  chi-square .  (1980) sampling  f o r such  folded  Jong e t a l . i n which the  contacts  but  test drew  data  order  (Yarranton  & Harper  usual  fold array  between t h e  1977,  a  been t o  triangular  aggregate  of c o n t a c t  method of a n a l y s i s  straightforward  the  Jong et a l  treatment  Subsequently,  resulting  j without  incorporates  table.  improper  is  previous practice  were d e t e r m i n e d .  1979b) have a n a l y z e d procedure  matrix,  1979a,  incorrect of  a 2 X  attention and  a  tables.  ( i n press) designed distinction  to  presented  contingency  2  a more  between  the  72  TABLE 9.  Count m a t r i x  are  respectively  the  number  species;  generated  t h e row and column  of ( i , j ) c o n t a c t  N i s the t o t a l  First selected spec i e s (point species)  2  -  n  i2  3  n  n  -  "2 1 31  n  n  ' ' •  23  * * •  32  •  TABLE  S  that  i n Table  choice i  • •  other  table . f o r  n . .  f 2 f 3  f  K  N  S  notation  s.-n. . 13  r  n  u  f  N-f•-f•-s•-s• +n..+n.. 13  S . 3  Total  *i £  3i  1  •l  (i ,j ) interaction;  other  -  n . .  s.  f  IK n„ 2K "n„ 3K  choice  -  3  Total  n  9. j  s.-n. .  Total  K  13  j  mate)  •  3  i  of  «  S  Second First  k; n „ i s number  -  2  -10. - C o n t i n g e n c y  follows  •  and s  taken.  K  K2  S  1  f  total  (contact  !3  •  Total  species . . .  •  n  the  3  •  R  is  of contact-samples  selected  1  sampling.  t o t a l s f o r species  pairs; K  number  Second  1 2  by c o n t a c t  3  N-f.-f. i  3i  N-S.-S. i 3  N  3  73  ordered  pairs  tabulated  (i,j)  i n an  unfolded  growth  forms and  dictate  that  different Jong  first  two  cover  a l . (in  the  densities  of  for v e r t i c a l  assumed  virtually  grasslands  as  essentially second  choice  present  choice  analysis be  after  for.  The  a  community  and  q  lateral  of  relative  i s the  position  test  i s c o n d i t i o n a l on often  a  calls  are  As  i n terms  the  relative  related s e t of  de the  to  growth  parameters  which types  1966)  may  be  such  as  or  grazed contacts the  p r o p o r t i o n a l to the  nonfirst  the " p r o p o r t i o n a l i t y  (in press). are  be  for  implies that  been c a l l e d  selecting  D e t a i l s of  presented  there  data  and  will  the  cover  relative i n the  ordered  pair  (i,j) ,  not  by  i,j=l,2,...,K; i ^ j  i  vertical  is  will  such s i t u a t i o n s  which  in  species  strategy.  vegetation  In  plane  association  the  This  second  Yarranton  study.  J o n g e_t a l .  i  the  different  to  set  are  below..  probability  is  second  (e.g.  P qj/(l-q ), p  data  Variations  i n some v e g e t a t i o n  assuming p r o p o r t i o n a l i t y , i s given  where  this  related  This  T h i s has  random  summarized  first  probabilities  de  the  sampling  s t r u c t u r e i n the  probabilities.  hypothesis"  the  level.  in a  9.  the  random a s s o c i a t i o n h y p o t h e s i s  communities  occur  and  probabilities  argued,  negligible  i n the  amongst  specified  s p e c i e s , and  contact  lichen-bryophyte  in Table  choice  have  the  accounts  only  above  press)  various  at  as  second  s e t s of p a r a m e t e r s : of  zero  matrix  and the  parameterization of  ( j , i ) i s maintained  percentage cover  under  et  and  frequency  of  species  k in  g r o w t h d e n s i t y of  species  c a n o p y where c o n t a c t  occurs.  a l l other  restrictive  associations assumption  assumed  w h i c h can  k  the at This  random. be  avoided  74  under  the  proportionality  that  q  = p  (i,j)  becomes  and  the  hypothesis.  probability  This  of  hypothesis  selecting  the  states  ordered  pair  p.p./(l-p.)• i j i The  estimation  of  maximum  counts,  e  using  i t e r a t i v e procedure  an  press).  under  the  To  Pearson  either  measure  g o o d n e s s of  the  EE  2  Significantly hypothesis  large  used  ij  (n;  values  to  Because  the  chi-square  legitimately  arbitrary should in  be  less  at  that  least  than  . ) / e . 2  the  De  i s accomplished  Jong  et  al.  (in  .  argues  data  against  values,  i  matrix  and as  a  'large-sample'  practice  set  rare  in t o t a l ,  the  hence  method, i t  a  counts  is  whole.  where t h e r e a r e  has  number of  Accordingly,  the  ID  in situations  40, c o n t a c t - s a m p l e s  cell  used:  e j  is a  expected  five.  model  chi-square  applied  expected  from random a s s o c i a t i o n  ID  technique  the  by  was  i n the  Conventional  limit  .-e.  construct  non-randomness  observations.  !3  of  for  be  outlined  deviation  evidence  cannot  random p a i r i n g  f i t statistic X =  likelihood  few  somewhat  i n each  species,  cell  occurring  were e x c l u d e d . f r o m  the  analysis. For  investigation  whole m a t r i x Table  10.  calculated for to  the  of  calculated  particular  c o u n t s was  first  pairwise  collapsed  associations,  into c e l l s  Expected  cell  u s i n g an  i t e r a t i v e p r o c e d u r e which mimics  overall test  indicate  pairs  of  of  departures particular as  C=1-P  and  counts corresponding  again,  from  a chi-square  randomness.  interest,  where P  to  i s the  an  index  defined  this table that  statistic  For of  significance  the  certain  was  by  were used used  species  association probability  was from  75  the  chi-square  significant  test.  The  sign  associations  was  residuals',  being  'standardized  (positive  obtained  or  from  negative)  the  t h e components of  sign  of  of  the  chi-square:  (n. . -e . . ) /Je. . iD iD D • 1  The  index  C therefore  association)  and  ranges -1.0  between  (maximum  Significant  a s s o c i a t i o n s (at l e s s  O + 0.95  C<-0.95.  and  +1.0  (maximum  negative  than  the  5%  positive  association).  level)  occur  when  RESULTS For global that  a l l 27  data  departures an  exact  from test  assumes t h a t o t h e r proportionality J o n g e t a_l. ,  signs  pairs  are  press)  chi-square  was  does not was  for  negative),  decided  inappropriate.  invoke  this  therefore all  assumption  used  pairwise  (P<0.05) p a i r w i s e a s s o c i a t i o n s ,  ( p o s i t i v e ' or  indicated  p a i r w i s e a s s o c i a t i o n s which  random  and  test  A c c o r d i n g l y , i t was  for i n d i v i d u a l  associations  Significant  overall  randomness.  hypothesis in  significant  s e t s the  to  The (de  detect  combinations.  together  with  are-, l i s t e d , i n T a b l e  their  1 "1 . f or a l l  surveys Table  11  predominantly pastures. which  An  especially Most only  that  negative  is  this  positive  typically  significant or a few  is  Agropyron  between  and  a  grasses  more p r o n o u n c e d repens  association.  involved  T r i f o l i u m repens  one  associations  and  exception  shows p e r i o d i c  associations  in  shows  -  Poa  Significant  grass  and  a  are  in older compressa positive  non-grass,  Taraxacum of f i c i n a l e .  a s s o c i a t i o n s were  'temporary',  successive surveys,  and  then  occurring  disappearing  76  TABLE  11.  Summmary  associations is  in  of s i g n i f i c a n t  the three pastures  the a t t a i n e d s i g n i f i c a n c e  testing  for  the  (+ o r -) o b t a i n e d  sign  individual  level  from  names a r e a b b r e v i a t e d  in  where more t h a n  Complete  i n each s u r v e y .  (to 3  interspecific  Species cases  (P<0.05) i n d i v i d u a l  the  to their  one s p e c i e s  s p e c i e s names a r e a v a i l a b l e  decimal  pairwise Each  value  places)  when  a s s o c i a t i o n together standardized generic  i n Table  5.  residuals.  name o n l y  i s represented  with  except  f o r a genus.  1977  PASTURE  SURVEY NO. SEASON  AGROPYRON - DACTYLIS  1  2  3  SUM  FALL  SPR  K005  -,000  AGROPYRON - HOLCUS  -.030  AGROPYRON - LOLIUM  -.034  AGROPYRON - PHLEUM  -.036  AGROPYRON  - PLANTAGO LANCEOLATA  AGROPYRON  - POA COMPRESSA  AGROPYRON  - RANUNCULUS  AGROPYRON  - TARAXACUM  + .023  AGROPYRON  - TRIFOLIUM PRATENSE  + .046  AGROPYRON  - TRIFOLIUM REPENS  + .000  5  6  7  SUM  FALL  SPR  SUM  FALL  .002  000  -.„039  -.000  -.000  4  -.006  SPR  .021  .001 -.000  + .000  + .000  -.001  +.000  +.014  +.005  +.000  + .043 +.001 + „007  -.034  -.000  -.042  AGROSTIS - LOLIUM  +.000  -.004  -.019  AGROSTIS - POA COMPRESSA  + .018  AGROSTIS - RANUNCULUS  + .000  AGROSTIS - TARAXACUM  + .000  AGROSTIS - TRIFOLIUM REPENS  + .000 + .000  + .005  DACTYLIS - FESTUCA  .002  DACTYLIS - HOLCUS  -.007  DACTYLIS - LOLIUM  -.000  -.003  -.000 -.001  .004  DACTYLIS - PLANTAGO LANCEOLATA DACTYLIS - POA COMPRESSA  9  + .038  AGROSTIS - DACTYLIS  DACTYLIS - PHLEUM  8  -.000 -.000  -.004  -.000  -.000  -.001 .050  -.001  +.000  +.000  -.024  1977  PASTURE  SURVEY NO.  (CONT.)  SEASON  1  2  3  SUM  FALL  SPR  4 SUM  5  FALL  6 SPR  DACTYLIS - RANUNCULUS .  -,003  --009 + .005  DACTYLIS - TRIFOLIUM PRATENSE  .002  +.000  -.000  +.000  -.005  -.001  SPR  + .000  +.000  +.015  .000  + .003 + .002  FESTUCA - TARAXACUM  + .000  FESTUCA - TRIFOLIUM REPENS  -.009  HOLCUS - LOLIUM  -.001  HOLCUS - POA COMPRESSA  + .038  +.000  .036  -.004  +.013  +.000  +.001 +.000  +.000  HOLCUS - TRIFOLIUM REPENS  +.045  LOLIUM - PHLEUM LOLIUM - PLANTAGO LANCEOLATA  -.043  +.000  HOLCUS - RANUNCULUS  +.045  +.039  +.000  +.002  .005  -.005  -.000  .035  -.001  -.000  .017  LOLIUM - POA COMPRESSA  -.025  LOLIUM - RANUNCULUS  -.001 + .020  LOLIUM - .TARAXACUM  .000  +.038  -.000  -.005  +.013  +.001  +.019  +.000  PHLEUM - TARAXACUM  + .003  +.000  +.000  PHLEUM - TRIFOLIUM REPENS  + .009  LOLIUM - TRIFOLIUM REPENS  + .010  .007  PHLEUM - PLANTAGO LANCEOLATA PHLEUM - POA COMPRESSA  FALL  9  + .012  FESTUCA - POA COMPRESSA  HOLCUS - TARAXACUM  SUM  8  -.043  DACTYLIS - TARAXACUM  DACTYLIS - TRIFOLIUM REPENS  7  -.015  + .000 .005  +.001  +.003  + 043  +.,009  -.001  +.003  1977  PASTURE  (CONT.)  SURVEY  NO. :  SEASON :  1  2  3  4  5  6  7  8  SUM  FALL  SPR  SUM  FALL  SPR  SUM  FALL  POA COMPRESSA - RANUNCULUS  + .000 + .028  POA COMPRESSA - TRIFOLIUM REPENS  + .000  + .000  + .002  + .000  -.027  TRIFOLIUM REPENS - RANUNCULUS  TRIFOLIUM REPENS - T. PRATENSE  + .019  + .001  + .001  + .000  + .001  - .000  + .004  + .001  + .001  + .006  + .009  -.002  PLANTAGO LANCEOLATA - RANUNCULUS PLANTAGO LANCEOLATA - TARAXACUM  + .000  + .003  TRIFOLIUM REPENS - P. LANCEOLATA  TRIFOLIUM REPENS - TARAXACUM  SPR  + .000  POA COMPRESSA - PLANTAGO LANCEOLATA  POA COMPRESSA - TARAXACUM  9  + .048 + .000  + .000  + .014  1  2  3  4  5  6  7  8  9  SUM  FALL  SPR  SUM  FALL  SPR  SUM  FALL  SPR  -,028  -.000  _  .007  -.000  -.072  AGROPYRON - FESTUCA  -.029  - .005  AGROPYRON - HOLCUS  -.046  -.000  -.000  + .000  + .000  + .000  + .000  + .000  1958  PASTURE  SURVEY NO. : SEASON :  AGROPYRON - DACTYLIS  AGROPYRON - PHLEUM  -.026  AGROPYRON - POA COMPRESSA  -.001  •  -.013  - .022  -.007  -.006 -.045  + .043  AGROPYRON - RANUNCULUS  + .002  AGROPYRON - TARAXACUM  + .000  AGROPYRON - TRIFOLIUM REPENS  + .000  + .009  -.047  + .002  - .019  AGROSTIS - DACTYLIS  -.016 -.002  AGROSTIS .- HOLCUS AGROSTIS - RANUNCULUS  + .003  AGROSTIS - TARAXACUM  + .014  AGROSTIS - TRIFOLIUM REPENS  + .047  + .007 -.039  -.022  -.031  ANTHOXANTHUM - FESTUCA  + .037  + .000  ANTHOXANTHUM - PLANTAGO LANCEOLATA  + .000  ANTHOXANTHUM - DACTYLIS  ANTHOXANTHUM - POA COMPRESSA  -.034  ANTHOXANTHUM - RANUNCULUS  + .001  ANTHOXANTHUM - TRIFOLIUM REPENS  + .015  + .000  DACTYLIS - FESTUCA  -.000  - . 000  - .000  -.002  -.000  -.000  -„000  -.000  -.000  DACTYLIS - HOLCUS  -„000  -.000  - .000  -.049  -.037  -.000  -.000  -.000  -.000  DACTYLIS - LOLIUM  -.034  - .000  -.001  -.000  + .001  1958  PASTURE  SURVEY NO.  (CONT.)  SEASON  DACTYLIS - PHLEUM  1  2  3  SUM  FALL  SPR  SUM  FALL  -.000  -.000  .000  -.000  -039  -.005  -.009 +.001  DACTYLIS - PLANTAGO LANCEOLATA  -.009  DACTYLIS - POA COMPRESSA  + .000  DACTYLIS - POA TRIVIALIS  + .013  4  .000  -.000  -.000  +.000  +.000  +.002  -.000  +.000  +.000  5  6  8  7 SPR  SUM  FALL  -.000  -.001  -.009  -.046  +.000  +.000  + .000 + .026  DACTYLIS - TARAXACUM  -.002  FESTUCA - HOLCUS  -.005  +.017  FESTUCA - PLANTAGO LANCEOLATA  +.008  + .000  + .004 +.012  +.015  FESTUCA - POA COMPRESSA FESTUCA - RANUNCULUS  +.006  +.000  FESTUCA - TARAXACUM  +.006  + .000  -.002  -.008  + .000  + .000  + .000  -.031  -.001  -.000  + .002  + .000  + .000 +.000  +.009  +.000  +.022  -.021  + .000  HOLCUS - LOLIUM  -.012  HOLCUS - PHLEUM HOLCUS - POA COMPRESSA  -.034 + .000  -.011  HOLCUS - RANUNCULUS  LOLIUM - PHLEUM LOLIUM - PLANTAGO LANCEOLATA  -.023 + .007  HOLCUS - TARAXACUM HOLCUS - TRIFOLIUM REPENS  + .049  + .000  FESTUCA - LOLIUM  FESTUCA - TRIFOLIUM REPENS  SPR  -.000  DACTYLIS - RANUNCULUS  DACTYLIS - TRIFOLIUM REPENS  9  +. 001  -.004  -.005  +.013  + .000  + .008 +.007  +.000  +,002  + .002  + .000 -.016  +.010  -.000  + .007 + „006  -.000  + .001  oo  1958  PASTURE  SURVEY NO.  (CONT.)  SEASON  1  2  SUM  FALL  3 SPR  4  5 SUM  LOLIUM - POA COMPRESSA  FALL  SPR  -.035  -.005  8  7 SUM  LOLIUM - TARAXACUM LOLIUM - TRIFOLIUM REPENS  +.001  +.031  PHLEUM - POA COMPRESSA PHLEUM - POA TRIVIALIS  9  FALL  SPR  -.024  +.000  + .018  + .007  LOLIUM - RANUNCULUS  6  +.000  +,000  -.000  -.015  +.004  +.000  + .001  +.008 +.005  + .000  +.000  +.023  PHLEUM - RANUNCULUS  + .012  + .001  PHLEUM - TARAXACUM PHLEUM - TRIFOLIUM REPENS  -.030  POA COMPRESSA - POA TRIVIALIS  -.022  +.014  +.000  + .006 +.012  POA COMPRESSA - P. LANCEOLATA POA COMPRESSA - TARAXACUM  +.013  +.025  +.000  +.000  POA COMPRESSA - TRIFOLIUM REPENS  +.000  +.000  +.000  +.000  +.000 -.020  TRIFOLIUM REPENS - RANUCULUS  + .000  +.004  +.001 +.000  -.028  + .047  TRIFOLIUM REPENS - P. LANCEOLATA  TRIFOLIUM REPENS - TARAXACUM  +.017  +.000  +.006  +.007  +.000  +.002  +.002  +.013  . + .000 +.036  +.000  PLANTAGO LANCEOLATA - RANUNCULUS  1939 PASTURE  SURVEY NO. SEASON  1  5  U  3  2  M  F  A  L  L  S  P  R  AGROPYRON - CIRSIUM  4  5  6  7  8  9  SUM  FALL  SPR  SUM  FALL  SPR  + .000  AGROPYRON - FESTUCA  +.030  +.040  + .032  AGROPYRON - HOLCUS  + .003  -,001  -.023  -.020  -.008  -.031  -.031  AGROPYRON - LOLIUM AGROPYRON - PHLEUM AGROPYRON - POA COMPRESSA  -.028  -.007  + .021 + .000  AGROPYRON - TRIFOLIUM REPENS  + .009  AGROSTIS - HOLCUS  -.013  +.000  AGROSTIS - TRIFOLIUM REPENS  +.021  +.001  +.001  +.012  + .005  DACTYLIS - CIRSIUM  +.000  + .001  + .039  + .037 -.000  -.006  -.008  -.001  DACTYLIS - LOLIUM  -.022  -.005  DACTYLIS - PHLEUM  -.018  DACTYLIS - POA COMPRESSA  -.011  DACTYLIS - TARAXACUM  + .043  DACTYLIS - TRIFOLIUM REPENS  + .000  FESTUCA - POA COMPRESSA  -.000  -.002  -.000  -.007  -.047  .001  -.000  -.041 -.006  -.018  .018  -.006  +.000  +.001  +.007  -.039 + .046 +.000  +.000  +.000  -.039  FESTUCA - RANUNCULUS  + .000  FESTUCA - TARAXACUM  + .037  FESTUCA - TRIFOLIUM REPENS HOLCUS - LOLIUM  -.005  -.005  AGROPYRON - TARAXACUM  DACTYLIS - HOLCUS  -.008  + .014  +.026 -,,017  +.008  + .043 -.000  -.000  1939  PASTURE  SURVEY NO.  (CONT.)  SEASON  1  2  3  SUM  FALL  SPR  4 SUM  HOLCUS - PHLEUM  -.037  HOLCUS - POA COMPRESSA  5  6  7  FALL  SPR  +.013  -.020  -.023  -.000  +.001  +.000  +.000  + .026  -.000  + .010  -.000  -.000  -.000  -.005  + .027  LOLIUM - TARAXACUM LOLIUM - TRIFOLIUM REPENS  FALL  SPR  +.001  -.011  -.000  +.003  +.026  +.001  +.000  -.000  -.000  -.000  -.001  +.016  +.026  +.015  +.000  +.000  +.016  + .022  -CIRSIUM  LOLIUM - POA COMPRESSA  SUM  -.046  HOLCUS - TARAXACUM  LOLIUM  9  + .026  HOLCUS - RANUNCULUS  HOLCUS - TRIFOLIUM REPENS  8  + .000  + .000  + .000  -.012  PHLEUM - POA COMPRESSA  + .000  +.000  +.000 CO  + .012 + .022  PHLEUM - RANUNCULUS  -.020  PHLEUM - TARAXACUM  + .032  + .000  PHLEUM - TRIFOLIUM REPENS  +.003  -.014  -.006  + .048  + .000  + .035  + .024  + .000  + .000  TRIFOLIUM REPENS - CIRSIUM  -.041  -.017  TRIFOLIUM REPENS - RANUNCULUS  -.023  -.000  POA COMPRESSA - RANUNCULUS POA COMPRESSA - TARAXACUM  + .000  POA COMPRESSA - TRIFOLIUM REPENS  +.000  +.000  + .000  +.000  +.000  +.000  +.000  +.028  +.021  +.014  -.022  TRIFOLIUM REPENS - TARAXACUM  -.008  RANUNCULUS - TARAXACUM  -.037  85  and  sometimes r e a p p e a r i n g  had  notably  pastures  (Table  positive evident  and  had  had  Some  4 such  pairs  with  Figure  8.  the  increasing  pasture  different  pairs  of  of  more and  Likewise,  patterns  of  seasonal a s s o c i a t i o n lanatus  and  association  Poa  but  youngest  pasture  T^  (Fig.  but  this  seen  In  The  same t r e n d  pasture  constancy  illustrated  was found  in  with  between  association  4  the  between  (Fig.  8 b,d,g,i,j).  show a  similar  trend  community c o n s t a n c y .  in  pattern  the  of  species  8 a , c , e , h ) and  developing  pattern  was  1958  development  1977  the of  between  was  1939  in not  and  association  for L^  in 1958  in  the  glomerata  all  three  present  progressively  A  Holcus  negative  pasture,  associated  p a s t u r e s where a s s o c i a t i o n 8d).  no  positive  particular  •(Fig.  surveys  seasonal  seasonal  or  w h i c h were s i g n i f i c a n t l y  8f).  had  1939  negative a s s o c i a t i o n  increasing  spring  showed no  the  the  association  been  compressa  (Fig.  is  positive  r e p e n s were p o s i t i v e l y  surveys older  can  in a l l three  pastures,  and  of  especially  pasture  is further  5 d i f f e r e n t grasses  tendency  between  However, t h e  and  between  species  seasonal  younger  two  was  stability  age  stable  stable  repens  the  7).  notable  grass  this  period.  increasing  of  pasture  alternated  youngest  associations  pasture  age,  Trifolium  and  oldest  w h i c h were c o n s i s t e n t l y  association  of  pairs  The  (Fig.  Particularly  a  species  'stable'  increasing  reflecting  than  33-month s t u d y  negative  development  associations  association  associations  and  The  associations  occurrence  positive  surveys.  youngest p a s t u r e .  over  6 stable The  12).  paired  negative  pasture  temporary  negative  i n the  significant or  fewer  in l a t e r  i n the  less  perenne  fall two  variable and  T^  86  FIGURE 7. and and  Stable pairwise associations  negative b) t h e  (P<0.05)  H.  1939  in  officinale lanatus  -  broken  lines) detected  pasture.  a l l nine  i n a) w i t h  (positive  Each  surveys  lines,  i n a) t h e 1958 p a s t u r e ,  association (except  P=0.070 i n s u r v e y  - solid  P_^  was  compressa  5, and p_j_  i n b) w i t h P=0.083 i n s u r v e y 3 ) .  significant - T.  glomerata  87  1958 PASTURE  TRIFOLIUM REPENS  FESTUCA RUBRA  POA COMPRESSA  ,/  DACTYLIS GLOMERATA  TARAXACUM OFFICINALE  //  HOLCUS LANATUS  1939 PASTURE  DACTYLIS GLOMERATA  N  TRIFOLIUM  88  FIGURE  8.  selected  Course  the  study  fields  1982  are  sampled  association. P<0.05  conducted  over a p e r i o d  pairs was  in  standardized probability Closed  sown.  each  the  number  Survey data  as  graph. C=1-P  residual level  circles  (C>0.95 o r C<-0.95). are indicated  versus  o f 33 months from June  indexes c a l c u l a t e d  the  significance  at  field  represented  association of  showing t h e d e g r e e o f a s s o c i a t i o n f o r  i n d i v i d u a l species  since  sign  lines  attached  for  represent  the  years  from the t h r e e 1979 t o  Plotted with  of  values  the  March are  appropriate  and where P i s t h e  particular  pairwise  significant  associations  The months i n w h i c h  s u r v e y s were  a s M=March, J=June, S=September.  89 PASTURE  1958  1939  PASTURE  PASTURE  a) Agropyron Holcus  -1—I S  1 1—I M J S  1 M  —i i  i i i  J S  M J S  I I J S  1—I 1 M J S  i i i—i M J S  1—I—I M J S  repens —  lanatus  —i—i—i—i—i—i—i—i—i  M  J S  1 M  M J S  M J S  M  Agropyron  repens  Trifolium  repens  —  ~i—i—i—i—i—i—i—i—i J S  M J S  M J S  M  c) Dactylis Holcus  L  11 J s  21  22  ^—rM J  23  40  YEARS SINCE PLANTING  41  42  glomerata  lanatus  90 1958  PASTURE  1939  PASTURE  PASTURE  d) Dactyl i s Trifolium  I M  4  I  1—I  J S  I  M J S  I  I  M.I  21  22  1 S  23  1  1—I  M  J S  M J S  M .1 S  40  41  42  YEARS SINCE PLANTING  1—I—I  1  1—I  1 M  glomerat. repens  91 1977 1 .00  PASTURE  1958  PASTURE  e) Ho]cus l a n a t u s —  0 . 6 0 -1 0.40  Trifolium  c 0.20  z  z  0  <  ~  -0.20  X •r. -0.40 -0.60 -0.50- 1  . 00  PASTURE  T  0.80  «  1939  —1—1—1  1—1—i—  M J S  M J S  -1—r— J s  -1—1—1— M J S  -1—1 1— M J S  repens  92  1977  PASTURE  1958  PASTURE  1939  PASTURE  93  repens  which  were s i g n i f i c a n t l y  three  spring  less  variable  stable  in  1958  other  in  the  officinale the  surveys  for the  1939  the  1977  two  older  pasture.  showed a s t a b l e  pasture two  but  a  pastures  increasing  progressively  older  pasture,  but  was  and  positive  and  Taraxacum  positive association  pattern  8k).  the was  compressa of  in  association  p a s t u r e s and  behaviour  (Fig. of  in association  P_;_  temporary  stability  negative  of  The  association  o v e r a l l trend  associations  between  in  in  the  i s one  of  species  in  fields.  DISCUSSION The  reaction  organisms  is crucial  populations biotic  of  and  to  individuals the  the  understanding  communities.  environment  to  The  of  manner  consititutes  a  changes  1968).  species  assemblages  and  temporal  scales,  associations  with other  changeable,  and  that  species  communities duration  need  evolutionary  n a t u r e of very  variously  evolutionary be  imposed  changes  provide  of  the  (Cantlon spatial "Species  loose i n the  and sense  recombined  into  'evolutionary  almost  any  time';  time  scale  1976a).  interspecific  local  the  s p e c i a l d i s t i n c t i o n of  for on  with  commented,  and No  however  both  broad  is net-like  time".  r e s u l t s presented  over  other  of  part  are : predominantly  combined  occur  ecology  i n many s p e c i e s  (1975)  evolution  may  (Antonovics The  species  community  are in  Whittaker  of  coping  major  genetic  r e f e r r i n g to  the  of  environmentally-induced In  presence  scale.  evidence  association  Detailed  studies  discussed of  for by  the  transient  Whittaker,  on  developmental changes  a in  94  local  p a t t e r n s of a s s o c i a t i o n  i n communities,  from  e y e - v i e w " , have been r e c e n t and few i n number 1979,  Turkington  involves  more  & Cavers intensive  investigation  i n pastures  in  studies.  previous  findings  in earlier  preponderance in  nature  1979, S a u l e i sampling  which d i f f e r The  work,  results i.e.  of i n t e r s p e c i f i c  during  the study  associations  which p e r s i s t  same  time  than  summarizes  the  temporal  The p r e s e n t longer  more w i d e l y are  study  period  in  age  consistent  younger  period  do  a  et a l .  of than  with the  communities  have  a  a s s o c i a t i o n s which a r e temporary  more  period  (Aarssen  1981).  over  the " p l a n t ' s -  and  older  essentially  younger  communities  have  unchanged o v e r t h e  communities.  Table  12  p a t t e r n s of a s s o c i a t i o n s i n the t h r e e  pastures.  TABLE  12.  Numbers  associations different  (both p o s i t i v e  aged  temporary,  and n e g a t i v e )  seasonal  detected  and  stable  i n the  three  pastures.  YEAR OF  NUMBER OF SIGNIFICANT ASSOCIATIONS  PLANTING  Temporary  Seasonal  (P<0.05) Stable  1977.  52  2  0  1958 '  57  0  4  1939  35  1  6  Apparently, changeable' stability trends  of s i g n i f i c a n t  a s s o c i a t i o n s between and in  toward  pasture species  stability  s p e c i e s become l e s s  community  evolution  interrelationships. in interspecific  tends The  ' l o o s e and toward  a  developmental  associations (Fig.  11)  95  parallel  those d i r e c t i o n a l trends  percentage  cover  data  neighbour contacts patterns  from  however  quadrat  particular  neighbours  experience  of  picture  its  i s that  assemblage  of  relationship  of  flux,  to  a  proximate  of  transition  neighbourhoods consituent fixed  and  A  view  popular  Whittaker  and  context  species  of  be  best  evolutionary of  'parallel'  in  context  evolution 137):  ...  the  or  coadaptive  coevolving  species....  appear  adaptation  to environment  will that  the  evolve".  pattern,  Community  physiognomy,  the  fact  level  evolution  trophic  coadaptive associations  by  is  and  that  contact)  an  are  into a  propounded community of  a  by  is  the  community of  the  , assemblage  of  Through t h i s f o r the  evolution  community  characteristics perspective  (e.g.  stability,  trends  as  also means  diversity,  networks, e t c . )  recognizable they  community.  evolution  structure, or  pasture  in t h i s  characteristics  c e r t a i n deducible  dictated  community  Community  community-level  dominance,  follow  species....  most  them  evolution  and  as  of  placing  "The  interacting  well  which the  was  species.  will  random  state  physical  f o r the  community's  there  The  in  i n t e r p r e t e d by  (1971, p.  evolution  general  positional  constant  matrix  (i.e.  The  the  the  plant's  essentially  a  on  address.  community  Woodwell  entail  i s in  neighbours  of  individual  which  community  l e s s permanent  r e s u l t s may  theoretical  members  identification  an  in  data  vegetational  environment.  from  quadrat  The  gross  the  biotic  of  2).  the  i n t o an  representing  a  ordination  Chapter  translates  sampling  interspecific  The  4,  immediate  more  s h a r e a more or  must  (Fig.  i n the  will  spatial tend  through  community-level  to  time  products  of  96  evolution  and i n t e r a c t i o n a t  Woodwell  1971).  sentiment  arguing  structure  are  community  that  reflected  the  interspecific  sampling  the  study  model  focuses  how  major p a r t  of the f u n c t i o n a l  exists,  at  the  genetic  and s p e c i e s  u n i t may  components  of  phenomena.  Hypothetical  'colonization' community  the  w h i c h may  by g r a z i n g  model  be  traditional Attention  community  (sowing  cast sense  i s focused  the  fashion  a  the  as  with  premise  of  that  the  of  contact  i n t e r a c t i o n s from t h e (1977b)  has  of d i v e r s i t y b a s e d on t h e  level.  of  a d e q u a t e and t h a t a  plant  and  a  community  The p r o p o s e d model  integral  diversity  treats  interdependent  rather  than  as  traced  from  the  initial  of t h e p a s t u r e ) t o some f u t u r e  s t a t e of  events  animals,  of  context,  to  In a d d i t i o n , H a r p e r  are  be d e f i n e d  as  but  the f a c t  diversity  diversity  c h a r a c t e r i s t i c are  used  n o t be e n t i r e l y  intraspecific  'coadaptive  p r i m a r i l y as a c o l l e c t i o n  descriptions  species  community  The c e n t r a l  of  In t h i s  be  on i n t e r s p e c i f i c  conventional  in  similar  f o r t h below i s c o n c e r n e d  with  of t h e i n d i v i d u a l .  a  &  evolution.  set  i s i n keeping  method  maintained  echoed  that  may  t h e community  taxonomic  the  is  i n t e r a c t i o n and c o e x i s t e n c e ,  This  (Whittaker  s e l e c t i o n ...."  i n a l l the o t h e r s .  f o r community  considers  perspective  (1975)  here  present  model  genotypes.  level  the observed p a t t e r n s  p r o d u c t s of n a t u r a l  Ultimately,  stressed  Diamond  "...  changes  from  model  species  o r n e t w o r k s ' and c h a n g e s i n t h i s  by  qualitative  the  &  characteristic  associations  results  Cody  the  a  arbitrarily  although  description replacement  as a  separate  'subclimax'  i t i s not intended of of  on f i n e r - s c a l e e v e n t s  succession 'community occurring  in  that the  types'. in  local  97  neighbourhoods displayed  in Figure  1) When the both  within  and  which t o g e t h e r comprises  9 and  pasture  genetic  the  community.  2)  Local  with  the  (and  (gene  subsequent  (local  community community  the  diversity  (e.g.  variations  patterns,  stochastic  will  be  for different  random and  unpredictable  higher  greater  meeting  one  the  opportunities the  factors  affected  with  The  another  will  Growth  hoof  and parts  events  drift, for  etc.)  affecting  diversity  in  events  the  provide  g e n o t y p e s t o combine their  in  marks),  chance  genetic,  'sample'  (species  be  f o r new  neighbours  and i n a.  species  Stoloniferous  species  s u c h as  to  genetic),  the  more  'random a s s o c i a t i o n ' , i . e .  form w i l l  species  and  of d i f f e r e n t  for  rhizomatous  species,  or p l a n t  other  These and  bank  emigration  plants  genetic  (2A).  - to  opportunities  extent  climate,  community seed  and  of  manner.  probability  randomly.  and  the  mole h i l l s ,  and  stages:  i n the  responsible  species  diversity  there  the  in  on  genetic  mutations,  Species  recombine  3)  represented  grazing,  model i s  sown, a d e g r e e  i s imparted  diversity,  The  following  p l o u g h e d and  newly a v a i l a b l e s p a c e ) ,  change.  opportunities  the  f l o w ) , n a t u r a l d e a t h of  dispersal  represent  by  diversity  'initial'  management p r a c t i c e s ,  community.  described  species  disturbances  immigration  pasture  is initially  total  the  the  also  species  i n f l u e n c e the  'sample'  their  T r i f o l i u m repens s u c h as  Poa  and  (and  compressa  genotypes number  of  neighbours. to a ) are  lesser able  to  98  FIGURE 9. Steps  A q u a l i t a t i v e model  for pasture  i n t h e model a r e d e s c r i b e d  i n the  community  text.  evolution.  1 INITIAL  COLONIZATION  (when pasture i s ploughed and sown)  ASSOCIATION  100  migrate within  the  neighbourhoods, this  context,  itinerant  and  the  of  T"\_  and  initial  period  t h a n any  played  by  one  which  other  (1979c)  has  inhospitable ones.  regarded  greatly  In  as  an  influences In  the  the  present  i n more s i g n i f i c a n t a s s o c i a t i o n s 'temporary')  (Table  species  and  be  associations.  were  species  may  which  involved  stoloniferous  Harper  species  random  r e p e n s was  from  happen upon c o m p a t i b l e  stoloniferous  (P<0.05) (most of  &  eventually  species  propensity study,  community, p e r h a p s e s c a p e  has  11). been  during  This  the  study  influential  suggested  been d e v e l o p e d by  by  role  Turkington  Aarssen  et a l  .  (1979). Depending these  random  predicted  3A)  on  The  as  (Aarssen  association  perhaps  may  (either  reform) respond  together  and  those which  competitive  exclusion  of  3B)  and  association  different .  be  and  compatibility  be  but  be  transient.  An  b r e a k down  (and  disturbances.  association  temporary  tend  Species to  may  periodically to  occur  become m u t u a l l y result  i f disturbance  halts  restores  the  vast  during  random majority  11.  s i g n i f i c a n t only may  in  may  i n s i m i l a r ways may  in Table  may  events  n e g a t i v e ) may  recurring  may  involved  1979):  interpretation applies  listed  species  r e s p o n d d i f f e r e n t l y may  process  This  associations  Seasonal  or  s i g n i f i c a n t negative  exclusion  The  of  the  significant  to disturbance  A  association.  e_t a l  be  because  of  three  positive  exclusive.  the  compatibility  associations,  follows  association  which  the  result  if  on  a two  seasonal species  basis. have  101  asynchronous  life  cycles  have some d i f f e r e n c e coexistence, 1967).  in  have  Mechanisms  (temporal their  partitioning).  ecologies  that  "ecological  combining  permitting  coexistence  may  environment. were  3C)  pasture  in  study  Stable  developmental  (Figs.  7  habitat.  The c o n t a c t  association  at  they  the  Such an a s s o c i a t i o n common  association preventing usually the  is  together  seasonal  fluctuations in associations  8 d , i ) and one i n t h e  thought  which  to be  than  to  (e.g  however  the  species a  reveals  present  a r e 'found  common  of i n d i v i d u a l  on  a local  i t means  exclusion.  type  that  Species  see  reviews ability  two s p e c i e s  to  exploit  both  species  by Grubb  experience.  If this is  which c o e x i s t  aire  they  exploit  (1977) a n d Werner  may t a k e on s e v e r a l  of c o n t r a s t i n g  total  growing a l o n e .  be  something  or p h y s i o l o g i c a l the  of  fine-scale  scale.  i n some w a y ( s ) i n w h i c h  combining that  in  a  a r e an  i s s i g n i f i c a n t l y p o s i t i v e may a l s o  some m o r p h o l o g i c a l able  level  requirements  to d i f f e r  Ecological  scale,  'adapted'  i.e.  associations  of the p a s t u r e s  critical  competitive  effectively  (Fig.  On a g r a n d  are  stable  b a s e d on t h e n o t i o n respect  Such  seasonal  s a m p l i n g method however  site  environment  (1979)).  two  significant  feature  and 8 ) .  because  to  study,  term  to the  may be s i g n i f i c a n t and p e r s i s t e n t ,  together'  due  short  ( H a r p e r 1964,  contribute  (3B.1).  their  (Fig. 8 f ) .  association.  important  by  t h e 1977 p a s t u r e  The a s s o c i a t i o n  stable  mediated  In t h e p r e s e n t  detected  1939  be  which  permits  ability"  m a i n t e n a n c e o f d i v e r s i t y i n t h e community differences  Species  habit  character,  environment  forms with will more  A resource-based  1 02  selective their  advantage  may be a f f o r d e d  complementarity d i c t a t e s  proximity Positive  to  may a l s o  between s p e c i e s  (and s i g n i f i c a n t )  some d i f f e r e n t i a t i o n consequence immediate terms  that  of  Harper  range  individual perhaps  one  acting  the  community,  community  in  A  neighbourhoods  superior  do ' f i t  in  constraints  rivals  random  which on  not  there  a  variability  ' f i t  range  will  a r e not r e s i d e n t s .  of  (with  .oust  within the  in', i.e.  diversity  may  i n ' a r e those c a p a b l e of immediate  will  or genotypes).  neighbourhoods  genetic  fitness  provide  association  species  genetic  in  of  found together as  reflecting  from c e r t a i n  reduction  in  of c o m p e t i t i v e  presumably  (i.e.  different  do  other's  an a d v a n t a g e  each p o p u l a t i o n  in a  existing  or  genotypes which  overbearing  random  those genotypes which  incompatible. Those  on t h i s  each  the  i n nature (3C.1).  variability  many  into  with  an u n d e r s t a n d i n g  species w i l l  Within  to persist  of  scale  threats  that  beneficial  may be a r e s u l t o f  enter  to quell  remain  genetic  abilities  any  Selection  of  rarely  a  o r commensal i s m .  spatial  diversity  a s e x p e c t e d by c h a n c e ) .  a  from  will  by  T h i s may c o n f e r  between  of s t a b l e  4) Many a s s o c i a t i o n s  be  association  (1967) has s t r e s s e d  differences  'explanation'  t h a n e x p e c t e d by c h a n c e .  horizontal  i f i t serves  and  t h e y may be f o u n d g r o w i n g i n  such as mutualism  negative  on a  association  encouraged  t h e two s p e c i e s  fitness  the c r i t i c a l  often  be  sphere of i n f l u e n c e .  exclusion.  the  that  a n o t h e r more o f t e n  association  interaction Stable  one  by s u c h an  are  result. residing  n e i g h b o u r s do n o t impose -  neighbourhoods  where  1 03  5)  The  above  compatible genotypes hence  selection  within  local  before  further  process  l e a v e s genotypes which are  neighbourhoods  selection.  contributes  to  may  the  Having  its  potential;  The  steps  during  t h e e v o l u t i o n of  the  community.  random  associations  significantly  full  remain  stable  As  selection  different 1922,  results  segments of  Turkington  Harper  they  in  association  chance.  Species  deviating  significantly attract  recognizable the  which  between  continuously i s that may  if (see  the  result  all  the s p e c i e s .  7)  As  more  community should  is  and  from  'biotic  3C  theoretically  community  matrix  be  above).  evolve.  Biotic  may  s p e c i e s r e g a r d l e s s of often  than  Harper  and  may  biotic  approaching  the  c h a r a c t e r i z e d more and  of u n c h a n g i n g  interspecific  by  association  (e.g.  Fig.  8)  e x h i b i t , the  most  (1977b) has  specialization  whether  expected  a more p e r s i s t e n t  attention  to  (Turesson  proposed  of a c l o s e e v o l u t i o n a r y c o a d a p t a t i o n  continually  become  ecotypes'  random e x p e c t a t i o n  ecotypes.  more  some  selection  p a t t e r n of a s s o c i a t i o n s i n o l d permanent p a s t u r e s  t o be  species  demonstrates  others  negative)  more  develop  particular  biotic  result  but  1979c)  evolve  however  or  of  and  i n members of a p o p u l a t i o n a d a p t e d  may  found  The  resident  coexistence  5 occur  or b e n e f i c e n c e  specialization are  2 through  t h e community,  &  permit  i n t h e model  random,  involves niche d i f f e r e n t i a t i o n  6)  circle  (positive  the  maintenance  diversity. feedback  come  This  than  more  is  that likely  involving  i s evolved,  'subclimax' more by  a  the  which 'fixed'  a s s o c i a t i o n s (some  1 04  of  which w i l l  fully  be  significantly  realized  decrease  because  in genetic  genes).  the  number of  opportunities  and  recombine  in association  several  transfigured  collection  of  specialized abiotic  there  are  younger  the  an  w h i c h may  are  communities  less  the  random  severe  be  and  combine  community  and  diverse  of  largely  threats  the  associations  on of  response  i s more p r e d i c t a b l e .  temporary  that  superimposed  then,  a  genotypes,  genotypes to  also  be  predicts  assemblage  r e s u l t s i n t o a broad  Only  i n t h i s way  can  may  drive  model.  stable  the  positive  negative  to  Hence, than  however  the  species  heterogeneous  gained  be  and  in  habitat.  showed v e r y  c o r r e l a t i o n with  i n t o the  the  of  leads  interactions. mechanisms  which  T r i f o l i u m repens  prominence  i s noteworthy  of  stable  (Fig.  8).  amongst  forms and  h e n c e p e r h a p s s i m i l a r ways  little  site  from t h e  cover  may  requirements soil  variation within  species  such  Negative a s s o c i a t i o n  different Results  species  in  expected  environment. have  species  .prouiinence  between g r a s s e s  s i m i l a r growth  exploiting if  The  may  t h e o r e t i c a l context  particular  i n s i g h t be  associations  grasses with  of  associations  Negative a s s o c i a t i o n  little  into  significant  to a c o n s i d e r a t i o n  2)  model  and  never  r e s u l t may  coexisting  essentially  neighbourhoods  next  result  net  is  communties.  Placing  of  species  older  exclusion  fewer  The  i s r e s t r i c t e d because  ecotypes In  in local  for  an  genotypes  ecotypes.  disturbance  2.  stage which  i t e r a t i o n s , the  from  biotic  competitive  step  - a  d i v e r s i t y ( i n t e r m s of  not  is  After  of  stable)  (Table  analyses fields  7).  as  also in  a  (Chapter and  very  105  Turkington association  of  found  5  of  of T.  their  factors  genotypes,  of  association  neighbouring Very  j  -  out'  grass  from  i n an can  associations forms t h e  as  said  and  line  of  interest  repens  These  positive  generated  h a b i t of  i s the  T\  and  to  selection  by  the  a  complex  stoloniferous  n e i g h b o u r h o o d s of grasses  environment  by  grass  a close  provided  by  the  t h a t e a c h of  four  exhibited  about  precise  different  the  data  biotic  species  nature  of  alone.  d e c i s i o n s t o be  or a f f i n i t y  of  species  Contact  sampling  made  of  are  i n subsequent  :  i for j , versus  procedures. the  species i  concerning'the  i n terms o f p r o c e s s e s  experimental what  of  random a s s o c i a t i o n  permitting coexistence inquiry  this  related  (1979c) f o u n d  sampling  of p a t t e r n s  to  of  be  respectively  permit  natural h o s t i l i t y  question  evidence  old pasture.  be  using c o n t r o l l e d  an  between  a f f o r d e d to the  repens  contact  i t does not  may  compatible  Harper  to  8 b,d,g,i,j).  nitrogen-rich  T\_  clear  positive  perenne  of p a r t i c u l a r  'wandering'  &  No  ability'  four  Interpretations  possible the  the  a stable  L o l ium  association  t o d e t e c t d e p a r t u r e s ' from  relative i.  of to  little  interactions  and  the  clones  specialization  serves  'combining  Turkington  different  and  repens  a beneficence  with  and  (Fig.  T\  'seeking  and  study  grasses  involving  repens,  legume.  repens  attribute  growth c y c l e s .  involving  some form  (1979b)  stable positive  different  associations for  T\  i n the p r e s e n t  development and  Harper  between  asynchrony was  &  mechanisms  in these chapters.  j  for  are  only  This  poses  producing  pastures?  This  106  CHAPTER 4  COMPETITIVE RELATIONS IN DIFFERENT AGED PASTURES A MIXTURE D I A L L E L OF F I V E SPECIES  1 07  INTRODUCTION  "The f a c t t h a t o r g a n i s m s l i v i n g in different places a r e d i f f e r e n t i s e a s y t o e x p l a i n by W a l l a c i a n f o r c e s . The question of how so many s o r t s of o r g a n i s m s a r e a b l e t o p e r s i s t t o g e t h e r i n t h e same ' p l a c e ' i s much more d i f f i c u l t t o answer, i s much more i n t e r e s t i n g ; i t demands biotic interpretation and a Darwinian s o l u t i o n " ( H a r p e r 1977b).  Traditional plant  intraspecific  s p e c i e s have been d o m i n a t e d by  concentrating populations  on in  environmental Hiesey  1948,  1970,  1971,  factors Kruckeberg Davies  differentiation 1937), o t h e r Simpson  habitat  Turesson  1951,  on  f o r c e s and  in  fields  perspective  the  this of  &  Other  &  recently  1966,  have  &  the  Keck  1976),  ecotypic  in  (Kemp  (Solbirg overall  abandonment  (Lovett  coexistence  and Doust  however  i n t e r a c t i o n s as of  a  &  1979,  to grazing  Wilson  point  abiotic  Snaydon  shown  of  focal  amongst  & Strain  stages  of c o m p e t i t i v e  intraspecific  Bradshaw  from human a c t i v i t i e s  species  have  Clausen  in relation  to a grassland  intraspecific  1922,  studies  different  represents  of  B r i g g s . .1980), c h a n g e s  (Hancock  role  differences  Teramura  within  perspective  qualities  1973,  between p o p u l a t i o n s  concerning  attention  Jain  Snaydon  of a woodland  Questions  large-scale  (e.g.  &  differentiation  'Wallacian'  different  1982).  conditions  a  to  1.974,. Warwick  conversion  document  and  forms, of d i s t u r b a n c e  agricultural  Only  local relation  Snaydon & D a v i e s  &  s t u d i e s of  in  of the  1981). center  selectional 'Darwinian'  differentiation.  efforts  been  differentiation  made t o associated  search with  for  and  biotic  108  specialization reported that of  in  population  appeared  and  more  vegetation  Snaydon  Dactylis  (in  populations  (1979c)  , Holcus  when  collected  repens  in  Harding  less  is  that  genetic  changes r e f l e c t i n g  biotic  such  environment.  differentiation  may e x i s t  single population  (e.g.  of  be s p e c i a l i z e d  t o each o t h e r  t h e same two s p e c i e s in  Snaydon  multispecies  1978,  (Allard Joy  interactions,  populations  did  & Harper  neighbouring Martin  specialization of  Erodium  The i m p l i c a t i o n o f t h e s e is  adjustment  a  consequence  such  intraspecific  between p o p u l a t i o n s , Turkington  & Harper  pair  in relation  of  to other  1969, Remison &  underscores  of  (specialization) to  & L a i t i n e n 1980). this  than  specialization in  for biotic  & Adams  pairs  a higher  Turkington  Some s t u d i e s have i n d i c a t e d t h a t a s p e c i f i c may  ;  1 ha i n s i z e .  Moreover,  not o n l y  odoratum,  had  specified  than  differentiation local  ecotypic  together  population  to  Remison  perenne  site  sites.  cicutarium.  findings  a  same  experiment  allopatric  E^  involved.  Anthoxanthum  to different  pasture  attempted  demonstrated  of  fine-scale  i n response  and  have  elements  from d i f f e r e n t  versus  obtusiplicatum  within  1974, Watson  (1981) a l s o showed e v i d e n c e  sympatric  the  1969, L i n h a r t  the  in  species  constitution  1978)  from  plant  the p a r t i c u l a r  l a n a t u s and L o l i u m  grown  species within a single &  the b i o t i c  Some s t u d i e s have  single  studies  populations  demonstrated  Trifolium  Other  collected  yield  populations  in a  (Watson  Snaydon  in  glomerata  combined  1978).  precisely  differentiation  of  differentiation  - i n Antonovics  pinpoint  to competition.  t o be a s s o c i a t e d w i t h  the surrounding  1974  response  but a l s o 1979c). genotypes genotypes Snaydon  In the c o n t e x t of  the  modus  operandi  109  of  " b i o l o g i c a l accommodation"  1979)  in  communities  perspective The  of  -  species  neighbours,  Previous  left  in  more  combining  implying  an  niche  measure  of e s c a p e  apply  for  1978).  evolved  evidence  for  grasses  involving  combining  ability  in  the  study  grasses,  so  the  to  a  :  or  clover.  test  simply  effects  of  distinguish  whether  such  selection in  the  to  a  for  improves  pasture  white  this  for  it  ability  about  is  possible a  (niche s h i f t ) ,  through  an  and  the for by the to  consequence or  improvement  r e l a t i v e c a p a c i t y t o r e d u c e t h e a v a i l a b i l i t y of  competitor.  in  evidence  not  as  however  (specialization)  is  of  ecological  attempt  is  to  ( i n Snaydon  clover  information  fitness  seems  specialization  local specialization  relative  species  interpreted  selection  whether  some  population  been  reciprocal  combining  that  & Snaydon  no  1977b)  confers  result  T h e r e was  competitors,  1964,  This  between  without  in their  interpretation  has  of  specialization  (Harper  the  the  of  selection  behaviour  one-way a d a p t a t i o n  for ecological  which  in  1977b).  Furthermore,  selection  This  above  certain  reciprocal  of  with  s t u d i e s of  environment  biotic  of Remison  process  (Harper  previous  their  of  'coadaptation'  i t i s not  coadaptation  data  described  local  evolutionary  precise  displacement.  specialization  repens  to  ability'  in mixture.  example t o t h e  Trifolium  an  compatible  from c o m p e t i t i o n  together  Biotic  the  interpretations  'ecological  higher  unanswered by  concerns  concern  yield  of  which s p e c i e s a d j u s t  resulting  presence.  nucleus  McNaughton & Wolf  coexistence.  specialization  m e c h a n s i s m ( s ) by  1968,  the  c r u c i a l question  biotic  (Saunders  resources  1 10  The respond  best r e f l e c t i o n reciprocally  involving former  selection  at  through  natural  neighbours  different  elucidate  the  reciprocal  of  the  most  objectives One  in  a  a  approach  with  1967,  Trenbath  t h e two  information competition. population to  different  homeostasis'  for  a  their  an  specialization  the  the  i s only  responses  established  biological requires  between  time  or i n  ontogenetic  accommodation a s t u d y of t h e  neighbours  i n a community.  in  a  changes  for  several  These  are  the  amongst  the  pairs  of  and  yields  1975,  1978).  Both  s p e c i e s and of  interesting  a stable  (Jaquard &  and  combine  s p e c i e s a r e grown  can  the  Caputa  as m i x t u r e s  yield  their  are  differential  t o t a l y i e l d provide  reciprocal t o ask  behaviour the  o b t a i n e d from 1970,  effects  whether a  i n f o r m a t i o n on be  in a l l  stands (Norrington-Davies  or s e n s i t i v e  Such  mixtures  members  their yields  as p u r e  analysis  associates.  the  In t h i s e x p e r i m e n t a l d e s i g n , c a l l e d number  component  exhibits  mixture d i a l l e l s  into  no  investigation.  pairs.  It i s also  of  i n nature, yet  change t h r o u g h  i s to i s o l a t e  analysis',  contrasted  between  investigations  p r e v i o u s e f f o r t s have been made  species  combinations.of  1968,  no  neighbours  to u n d e r s t a n d i n g the r e l a t i o n s h i p s  in a l l possible  possible  with  This  of the p r e s e n t  'diallel  biotic  r e s p o n s e s between n a t u r a l dominant  which  Insight  the r e c i p r o c a l  of  framework.  members of a community them  made.  community  process  in  interact  been  producing  Furthermore,  community-wide in  has  communities  relationship. to  actually  s t u d y i n g how  aged  way(s)  o t h e r must come f r o m  which  this  mechanisms  possible  the  to each  individuals  attempt  of  of  particular in  response  'sociological the a n a l y s i s  Rousvoal  &  of  Gallais  111  1973). using  The  a variation  dominant The  principal  information  of  species  biological to  above  the d i a l l e l  in  aim  i s sought set-up  the d i f f e r e n t  i s to t e s t  accommodation  i n the p r e s e n t  involving  5 of  aged p a s t u r e s  f o r the  occurrence  during pasture  community  c h a r a c t e r i z e t h e p o s s i b l e mechanisms t h a t may  the  under of  study most  study.  developing  e v o l u t i o n , and be  involved.  METHODS All were and  p o s s i b l e 2-species  collected  from  Poa  Dactylis  compressa  1980,  25  and  glomerata Trifolium  neighbouring  arrangement  shown  by a s p e c i f i c  basis  for  the  plants Co., 25  from  a pasture  times. the  field  British  at  .  In t h e  10.  Since  upon  the  and  planted  25  from  perenne , of  May  r a m e t s of i t s plots  genet  i n the  type  was  for  Ramets of  each  source  (Richardson's  of Seed  in a l l possible species pairs  m a p a r t and  experimental  field  'natural were  of  neighbours'  replicated  arranged  station,  no  composition  a stock  supply  A l l mixtures  1.5  five  s p e c i e s , t h e r e was  This represented  'zero'.  were s e t up  seed  pastures  week  field  each  genets  following  first  genotypic  obtained  aged  , Lolium  x 25 cm  natural neighbouring  as a b o v e .  the  the  lanatus  i n 25 cm  a commercial  of age  Plots  for  t h e s e -were t h e r e f o r e omitted.-  d e r i v e d from  ramets each,  2,  were p l a n t e d w i t h  above s p e c i e s were a l s o  B u r n a b y , B.C.)  three d i f f e r e n t  repens  genet  deciding  • m o n o c u l t u r e plots,*  the  , Holcus  in Figure  defined  of n a t u r a l n e i g h b o u r i n g  in Chapter  ramets of e a c h genet  natural  of  e a c h of  c l o n e d , as o u t l i n e d  species:  pairs  3  randomly i n  University  of  Columbia.  Successive  harvests  were  taken  by  placing  a  square  11 2  FIGURE  10.  positions lines placed  Planting for  indicate  25  ramets  of  in the mixture d i a l l e l e a c h o f two  t h e s i z e and p o s i t i o n i n g  over the p l o t  collected  arrangement  at each  within  harvest.  species.  The  of the h a r v e s t i n g  which the v e g e t a t i o n  was  showing dashed frame  clipped  and  113  5 cm  •  o f"  •  o  j •  o I  •  • o I  o  o  J  I • I  o o  •  o •  o  •  o~[  o  o  >  •  •  •  I• o  •  o  o  <  j •  oI •  o  I• o i  • • • I• o o o • • • I• o  o  o  I  114  harvesting  frame o v e r  vegetation  inside  clippings and  the  planting  The  the  3-4  of  April  mid-July  1981,  harvest,  the  collected, weights  dried  f o r each  were d e t e r m i n e d . involved  of  approximately  was  more  until  first  ground  weighed two  and  the  taken  total  i n the  In  & Guire  first until  the  final  plot  were  cummulative  dry  t h e one  used  at  Negligible  p a r t s w i t h i n each the  after  week i n t e r v a l s  components o v e r  (Fox  weeks  were  harvest.  A n a l y s i s of v a r i a n c e  a MIDAS p r o g r a m  10  h a r v e s t i n g resumed  the  3  November.  c o n t i n u e d a t 3-4  after  the  taken  harvests  intervals  above  and  clipping  dried  and  total  and  separated according to s p e c i e s ,  w i n t e r and  year  10)  The  Four  1981 one  to  harvest  week  growth o c c u r r e d over  (Fig.  cm.  first  ramets.  approximately  plot frame  were c o l l e c t e d ,  weighed.  week  each  in  year p e r i o d  data  analysis  1976).  RESULTS It  is first  variability associates species  i n performance (Table  from  variation  interesting  13) .  a given  in  to the nature  Holcus  lanatus  nature was  of  the  and  of  lowest).  that  reflects  its  showed t h e i n the  repens  to the nature  each  21  of  second  . for.  a  t h e c o e f f i c i e n t - of sensitivity  in  lowest  and  different  'performance  the  associates  Trifolium  for  of  i s measured by  p a s t u r e where i t was showed  species with t h e i r  var i a b i l i t y age  generally  sensitivity  0 year  analysis  13  of e a c h  i t s a s s o c i a t e s (except  second  highest  field  Table  species  The  t o compare t h e mean p e r f o r m a n c e  its  that  community.  sensitivity  year  of  to  the  p a s t u r e where i t  generally  showed  the  i t s a s s o c i a t e s (except i n highest).  species  (and  Statistical  for a l l species  1 15  TABLE  13.  5 x 5  matrices  of p e r - s p e c i e s  replicates)  from t h e m i x t u r e  as  I n a) a l l ramets were d e r i v e d  ramets.  source  (pasture  clones and  diallel  yields  age ' z e r o ' ) .  o l d pastures  involving 5 species  In b ) , c ) and d) a l l r a m e t s  were  genets c o l l e c t e d  respectively.  from t h e 2, 21  Probability  levels  from t h e a n a l y s i s o f v a r i a n c e  of mean y i e l d s ,  F-test  f o r the e q u a l i t y of v a r i a n c e s  f o r each  species C.V. H.1.  from  a given' f i e l d  from a g i v e n  = coefficient =  compressa  field  age.  of v a r i a t i o n .  lanatus  ; L.p.  ; T.r.  = Trifolium  D.g.  species  with a l l when a l l  (*) a r e a l s o  shown.  = D a c t y l i s glomerata  = Lolium  repens .  are  and from an  Attained P levels  age a r e a v e r a g e d  Holcus  planted seed  given  associates  3  from a c o m m e r c i a l  of 2 n a t u r a l neighbouring  40 y e a r  (g) (means o f  perenne  ; P.c.  ;  = Poa  11 6  ASSOCIATES  PRODUCERS P.c.  T.r.  113.0 101.3 181.5 180.0  2.2 1 .0 1 .6 17.7  35.8 21.8 46.5 91 .6  145.9 31.4 0.22  1 44.0 42.8 0.30  5.6 8.1 1 .45  48.9 30.2 0.62  223. 1 179.9 369.9 448.3  76.3 212.9 143.6 144.7  106.7 35.7  9.7 16.4 11.4.  187.6 108.6  1 .7 29.5 70.8 81 .2  26.4  mean s t . dev. C.V.  305.3 125.3 0.41  1 44.4 55.8 0.39  109.7 62. 1 0.57  16.0 7.5 0.47  45.8 36.9 0.81  D.g. •H.l. L.p. P.c . T.r.  226.6 273.3 270.6 419.6  84.6  70.3 58.5  1 82.9 242.3 225.2  180.2 1 62 . 4  12.3 7.3 5.2 24. 1  3.3 2.2 11.4 59.4  mean s t . dev C.V.  297.5 84.2 0.28  183.8 70.7 0.38  117.9 62.3 0.53  12.2 8.5 0.70  19.1 27.2 1 .42  -•  150.5  .33. 1 ., .10.5. 73.3 13.8 • - • 11.2. 163.0 61 .3 36.6  a)  D.g. H.l. L.p. P.c. T. r .  mean s t . dev. C.V.  b)  c)  D.g. H.l. L.p. P.c . T. r .  . . D.g. ..H.l. ' d) L.p. P.c . T.r. mean s t . dev. C.V.  P (analysis of v a r i a n c e )  D.g.  H.l.  L.p.  116.0 1 09.4 257.8 181.3  121.6 116.7 1 79. 1 166.2  1 66. 1 69.2 0.42 •-  -  -  221 .4 291 .4 346.9 408.6  128.5 211.3 172.5  317.1 79.8 0.25  165.7 35.5 0.21  0.1230  P (equality of v a r i a n c e s ) 0.7743  -  -  -  Mean  —  102.1 36.3  —  124.2 57.5  —  1 26. 1 50.6  . • 5. 4 16.2 72.0 54.3 —  82.7 56. 1 0.68  18.0 12.5 0.69  37.0 31.4 0.85  124.1 43.0  0.6662  0.5167  0.3007  0.5596  0.9823  0.5271  0.9313  0.8254  0.9677  0.7431  (*)  117  averaged),  t h e r e was a  from  different  the  remarkable aged  consistency  pastures  in  amongst  their  plants  mean y i e l d s and  variances with a l l a s s o c i a t e s . Attention responses  now  turns  to  between p a r t i c u l a r  aged p a s t u r e s .  An i n d e x  reflection  the  of  neighbours.  the  neighbouring  of 'combining  reciprocity  Combining  differences  ability  in  reciprocal  s p e c i e s from  ability'  was d e v i s e d a s a  of c o m p e t i t i v e  i s calculated  different  effects  between  as  CA = Y/Y' where Y and Y' a r e t h e r e c o r d e d lower CA  and h i g h e r indices  have  combinations the  same  yielding  of a g i v e n  two s p e c i e s ) from  balanced  in  a value  their  .given  neighbouring,  parameters: changes  individual  Figure  1) c h a n g e s  type 11.  summarized  as  of r e c i p r o c a l  species  from  pair  by  the  total yields the  Trends i n Table  ( i . e . of V a l u e s of  are  are  more  combinations  are  encompassed  to t o t a l y i e l d CA  index.  by  aged  with  for  pastures  increasing  from  two.  and 2) t h e two  A comparison of  and CA i n d e x e s  different  i n the data 14.  competitive effects for  i n t o t a l y i e l d of the c o m b i n a t i o n ,  reflected  yields,  type  than  the  comparing  components  mixtures  i n the r e l a t i v e c o n t r i b u t i o n  components  pair  in  in  aged p a s t u r e s . two  of  to zero.  Changes i n t h e n a t u r e a  the  pair  (g)  respectively.  value  genet  different  that  yields  closer  relative  neighbouring  t o 1.0 i n d i c a t e  yields  components o f a m i x t u r e  strictly  CA c l o s e r  having  d r y weight  each  genet  i s presented i n  pasture  age  are  118  FIGURE yields type  11.  Comparisons  and c o m b i n i n g collected  case the  ability  from  y e a r s ) and grown  yields  field or  CA  indices  different  in competition  histograms represent four  of i n d i v i d u a l  ages  (with  indices),  component y i e l d s ,  (CA) f o r  aged  each  a mean o f 3 r e p l i c a t e s . to  those which  b a s e d on S c h e f f e ' s m u l t i p l e  comparison  o f v a r i a n c e s were n o t r e j e c t e d on u n t r a n s f o r m e d d a t a .  each  yield,  total  do n o t s h a r e a common  level  were p e r f o r m e d  In  In comparing  component  ( a , b, c o r d) a r e s i g n i f i c a n t l y d i f f e r e n t  homogeneity  pair  p a s t u r e s ( 0 , 2, 21 and 40  letter  the  genet  i n a mixture d i a l l e l .  respect  total  at the  test.  code  P<0.05  F-tests for  and a l l a n a l y s e s  YIELD (g) Ln  O  1 .  *-»  O  o  1  en  o o  o  1  1  YIELD (g) to  o o  1  Go O O •  -fc*  en O  o o  O O  Cn O  en  o  1  o o  1  Cn  O  1  to  o o  1  to  cr cr n  O  to to  cr  o o  O  2 tr)  jo >  TOTAL YIELD o o L_  CO  o o  1  to  o o  1  >  CA o  o o  o  1  0)  cr\  •  to 1  o  •  4^1  o •  1  o  o o  •  oo  1...  TOTAL  T3  L_  Co  o o  YIELD to o o  _L_  4^  o  o  cn jo >  >  CA o •  o o  to 1  _L_  CD JO m as z en  o •  4^  2  o •  o  •  oo  1  > > H G  cr cr  to  to 4^ O  00  YIELD (g)  YIELD (g) I—*  Oo  o  o o  to  o  _l  o  •  l_  to  o o  1  Oo O  o  1  o o  I  o o  ""TJ  a  1  o  ui  o  o  __J  I  1  L  to O  oo  o o  o  to to  to  o  o f o  O  jo >  o o  OO  o o  to  o o  _l_  >  CA  TOTAL YIELD o o  o o  I—I  _L_  _l_ to  o  to  JL_  o  _l_  J>  o o  tO O  _l  TOTAL  JO  _J  Oo  o o  l_  to  o o  _1_  o o  o  o  _l_  _l_  to  2  tn  > >  CA  YIELD  a to  n o  2.  W m  po PI cn en >  cr cr to  o  YIELD (g)  YIELD (g) to O  o  1  I  _L_  o  o  1  to  o  1  o o  o  o  VJ  O  I  to L/1  o  to  o  o  O  o  o  O  CD to  to  cr a  cr  to  o a  4*> O  TOTAL YIELD to O O  1  I-  1  1  l-  •  1  O O  1  P" >  Z >  4*. o  H C CD  TOTAL  CA o  Ln O  Ln O  O  O  1  1=  o  to  •  >— O  o o  O  to o o  1  o  1  1  o o  •  to  o •  1  o •  1  O •  GO  P3 CD  OO  73  1  tn z z  ?0  PI  PI GO GO  to  cr  ac  to  to  ab  be  >  cr  4*.  4^  o  H  CA  YIELD o o  > z >  o  YIELD ( g )  YIELD (g) Oi  o  O  _L_  _1_  I—  O 0  Ui o  1  »  tO  o o  o  _JL_  L_  to  o  _1_  Ol  o  o _JL_  JL_  Oi  o o  o  to  o o  1  to Oi  O  1  to  to  on  to m  o  o  1  On O  1  O  o  Oi  o  o  to  _1_  o _L_  o  cr  to  o  o o  OO  o o  1  1—1 On  O  1  o o  CO  CA  TOTAL YIELD  o  o  On  O  On  1  >O  On  8  _L_ tn z CO  ?0  m  co  cr  CO  >  to  to  o  > z > H  CA  TOTAL YIELD tO O  0)  m z z m  to cr  0)  cr  o  to to  YIELD (g)  o o  Ol  o  o  _L_  YIELD ( g ) to  O  o  o  o  o  to Ol  Ol  o  o  o o  Ol  o  1  1  o o  1  o B>  cr  fll cr  B)  cr o  to  cr  to  B)  B>  cr  o o  cr  o  2  O  73 in  CA  TOTAL YIELD  o o  _1_  GO GO >  o  •  Ol  to  o  1  _1_  o • 1  o  •  1  TOTAL to  o •  o o  GO  1  73  Ol  o  1  YIELD  o o 1  o ••  Ol  to  o  1  _1_  o  o  1  1  •  •  o •  00  73 m -o cn z  1  z  GO  GO  fl)  to  cr Bi  cr  cr o  m z z en  CA  m  B)  CD ?D  to  to  1 24  TABLE  14.  ability for  A summary of  indexes  different  (CA)  significant  and  genet  total  pair  (P<0.05) t r e n d s  yields  types  as  from  the  t h e age  of  i n c r e a s e s ( F i g . 11).  indicates  that.the value  increases consistently  and  decrease  with  indicate  a  significant means  that  combination  a  downward  increasing  fluctuating change w i t h the  pasture trend.  upward  with a  (-)  pasture  age.  occurs  from arrow  increasing consistent  Both arrows  dash  trend  An  indicates  age. A  increasing  indicated  i s excluded.  arrow  diallel  the p a s t u r e  were c o l l e c t e d  age,  combining  mixture  which they  pasture  in  together  indicates An if  asterisk the  no (*)  '0-year'  DACTYLIS/ HOLCUS  CA  TOTAL YIELD  DACTYLIS/ LOLIUM  DACTYLIS/ POA  DACTYLIS/ TRIFOLIUM  T-  4,  t  J,  t  4,  -  t  HOLCUS/ LOLIUM  HOLCUS/ POA  HOLCUS/ TRIFOLIUM  LOLIUM/ POA  LOLIUM/ TRIFOLIUM.  n n n n - n % - ±  t*  POA/ TRIFOLIUM  t  126  DISCUSSION  Recent  studies  competitive different 1982a,  r e l a t i o n s h i p s amongst successional  1982b).  These  'late-succession' respect  of  these  which  of s e l e c t i o n a l  differ  in  making  strong  basis  being  compared  for  successional making  species  from  differentiated  with  than this  are  species  considering in  these  any  In a l l  part i c u l a r  how  community;  species There  individuals  nature.  The  using  different  even  belong  a  series  reasonable the  communities to  considerable the  same  the  same  uncertainty  importance over  relationship  through  of  past  evolutionary  time  t o answer t h e between  in  a  two  part icular  s t u d i e s been b a s e d on i n t e r a c t i o n s  study of  been  i s a l s o not a  w h i c h were known t o be immediate present  a  have  '  competitive changes  communities  the competitive  been no f o r m e r a t t e m p t s  Nor have p r e v i o u s  in  the  about  1 980).  the  species  represent  of  i n t h e o r g a n i z a t i o n o f a community  of  problems  that  This presents  T h e r e have a p p a r e n t l y question  and h e n c e ,  groups  studies  inferences  from  f o r c e s from c o m p e t i t i o n .  t h e above c o n c l u s i o n s .  (see a l s o C o n n e l l  stages  more  1976, 1979,  t o be an  different  sequence.  competition  in  that  of  i s proposed  in species composition  compared  between  and  & Bazzaz  shown  are  communities  s t u d i e s , c o m p a r i s o n s have been made between  of e n t i r e l y  time  have  communities  relations  in  studies  from  (Parrish  dimensions of niche  'early-succession' product  species  maturity  communities  to certain  evolved  have d e m o n s t r a t e d d i f f e r e n c e s i n n i c h e and  i s an a t t e m p t  different  approximation  developmental  aged  t o address  these  pastures  which  of d i f f e r e n t sequence  neighbours  based  coexistent on  known  1 27  management h i s t o r y Using  this  as  w e l l as  approach,  competition  as  with  confidence  more  widely  interpretations  a selectional  different  ontogenetic  species  sensitivity  (variability  for  given  significantly  pasture  g e n e t s of a p a r t i c u l a r differences on  their  pasture  The the  in their  reported  the  of  biological selection  for  therefore  c o n s e q u e n c e of  to  focus  in  ability' have  An  the  the  overall  neighbours not  change  neighbouring clear  depending  in  the  by  ability Three  to  Species  the  in  respective competitive  assumption (or  that  may  a  stabilized)  e x c l u s i o n would  and  of  which  persisting  of  also  f e a t u r e s of  relative  based  t r a n s l a t e d as  their  increased  of  development  be  of  the  systems  relation  competitive  selection.  from e x a m i n i n g  in  means  in  of  the  mechanisms,  explanation  combining  of  c h a r a c t e r i z a t i o n of  fitness,  defined  is  made  showed  i n populations..  a decreasing  recognized  into  a t t e n t i o n on  resides  of  11).  some  another'  role be  another  community may  Conversely,  natural  however,  selection  a  3).  established  Natural  accommodation  natural selection  ability.  consequence be  one  attributes.  coexistence  may  ability  with  by  insight  no  did  r e p o n s e s t o one (Fig.  can  different  13).  species  natural  'combining  interaction  combining  of  organisms which c o n f e r  combining  reflected  of  accommodation  biological  (Table  biological  order  operation  attributes  have  In  pasture  a  the  showed t h a t  within  relative  lend  having  results  2 and  of c o m m u n i t i e s  to  of o r i g i n  of  comparisons  response)  age  pair  data  development  competition. on  age  in  The  species  with  concerning  composition  of  (Chapters  f o r c e i n communities  than  relationship.  any  e m p i r i c a l data  changes  be  be a  the  data  in  the  1 28  total  and  older  pastures  1)  respective (i.e.  A process  in combining  ability.  age.  Trifolium case,  in  probability Selection  by  an  stable  or  population other  than  species 3)  yield  most for  the  other  accordingly,  pairs, (CA)  the  a result are  exclude  by  demands  this  but  m i x t u r e were not  of  (e.g.  would be  This  that  the  reflected by  a  implies  a  each  resources  (or  trend.  data  an  increase)  i s t h a t a change  in y i e l d graphs  combining  bar  graphs  in F i g .  Poa/Trifolium  , while  different  In  decrease) 11).  with  The  changes  significant two  species  combining  age,  in  a  in F i g .  11).  one  usually  (i.e.  shows  the  is  ability  rarely  increasing pasture  one)  needed by  this  the  change  that  significantly  increasing  illustrated  yield  with  evenly  only  total  and  less  this  that  ( l e f t - h a n d bar is  In  remarkable  that  Holcus/Lolium increases  ability  the  is  feature  component of  on  It  opposite  and  other.  result  Dactylis/Holcus  an  selection.  ability).  the  has  Lolium  a f t e r s e l e c t i o n (accompanied  with  component  of  selection,  combining  selection.  increasing  both  partitioned  the  and  Dactylis population  against  after  yield  less  (right-hand  also  as  units  will  notable  one  consequence  changes  pastures  one  that  competitive  in behaviour  accompanied for  suggests  combination,  The  decline  decreases with  in t o t a l  before  a  ability  more  makes  by  14):  combining  increasing  displacement  Table  is reflected  for e c o l o g i c a l combining  increase  11,  pairs, Dactylis/Lolium  populations  that  2)  exclusion  (Fig.  from  species  resource  two  genet c o m b i n a t i o n s  two  older  available  between  In  This  become r e l a t i v e l y  y i e l d s of  'after selection')  of c o m p e t i t i v e  Dactylis/Trifolium, pasture  component  total  ability  y i e l d s in  increasing  pasture  1 29  age.  This  relaxed  result  due  has  component  to  both  1958 The  11  are  relative  reduce  the  availability  ,  thus  units  make  demands.  The  combination  and  1939  remaining  from  the expected  interesting  an  not  a  to  more  similar  ability  generally  is versa.  yield  also  In t h e  fluctuates  involving  declines  same w i t h  Although  interpretation  in  between  of  of  increase in by  As  but  ability  in  the  the  f & g). ability  combination  p a s t u r e age  these selection  suggests  yield  involving  11  (Fig.  of  i n the  total results leading  yield are to  one other  Holcus  In  the  the  total  h)  and  decreases consistent either  the  t o the  fluctuates  from  11  these  here.  in negative r e l a t i o n s h i p (Fig.  a  selection  selection  a decrease  combinations,  combination,  none of  forces  nor  these c o m p e t i t i v e  the Lolium/Poa combination,  increasing  Lo1ium/Trifo1iurn  the  trends (Fig.  outcomes o f that  Lolium, combining  the h i g h e s t v a l u e o c c u r s Also,  which  in  is indicated.  suggest  accompanied two  i n combining  combinations  the  balanced  f e a t u r e of t h e s e c o m b i n a t i o n s  component  i).  resources  is  ability  important  e x p l a n a t i o n - an  the  inferior  from a common s u p p l y on  alternative  an a l t e r n a t i v e  pasture.  the  show no d i r e c t i o n a l  c o m p e t i t i o n , i t may  However,  fluctuation  of  of  trend  combining  are  total  that  N e i t h e r a p r o c e s s of c o m p e t i t i v e e x c l u s i o n  traditionally  vice  the t r e n d s suggest  in  but c o m b i n i n g  combinations  interactions  and  been  pastures.  of e c o l o g i c a l  pressure  c o m p e t i t i o n has  ability  resulting  resource  f,g,h,i).  process  the  of  Dactylis/Poa the  that  improved  component  partitioning they  indicate  to niche divergence; rather,  selection  superior  does not  two but  oldest yield  is  in  the  (Fig.  11  with  an  competitive  1 30  exclusion  or  selection  niche  process  common n i c h e  These  be  is  which  with  they  relative  are  are  consistent  competitive  continuously  with  abilities  adjusted  so  a for  that  over  maintained.  data  associated  in  requirements  time a balance  may  differentiation,  suggest biotic  related  to  that  intraspecific  specialization three  differentiation  in response  alternative  to  competition  c o n s e q u e n c e s of  natural  selection: a)  increased  combining  differentiation b)  increased  competitive c)  Notice  both  a)  of  the  i.e.  'combining  ability'.  yield  increases,  permits  there  ability  has  superior  been  operating  accommodation  and  cases  Only case  the a)  i n a)  which  and  b)  coexistence,  in competitive  In c o n c l u s i o n , t h e  of this  the  two  i s that  the  coexisting  than  two  populations  have  however,  where  of  (Harper  biotic  1977b).  specialization  while  result  selection  total  'ecological'  reduces combining then  selection  of  interpretation  for  selection  abilities.  performance  previously defined  selection  competitor,  Selection  results  as  of  asymmetic  i t i s assumed t h a t  probability  an  'balancing'  from  i m p l i c a t i o n of  i n both  higher  combining  The  have a g r e a t e r  selection,,  the  niche  resources; resulting  b)  in  mixture.  from a  of c o m p e t i t i v e  and  differential  populations  before  ability  from  competition;  resulting  for contested  'unbalancing')  in  the  components two  (or  resulting  resource  combining a b i l i t y  combining  that  reduces  r e l e a s e from  abilities  reduced  improvement  and  ability  in  In  c)  in  the  ability. biological  operating  in  c)  exclusion. changes  discovered  in  the  combining  131  ability  and  naturally suggest  biotic  by  the  pasture  a clear  Turkington  t h e above Very  little  selection  that  evolution  differentiation) out  of t h e t e n  i,  j)  p a s t u r e age  of  interactions  several role lends  it  i n younger  dominant  community  support  evolution  to  (Fig.  9,  one  assumed result  ability  ability of  and As  a  (niche  11c,  with  Five e,  3).  h,  increasing  form  interpreted  the  differential  s u p e r i o r components this  site,  proposed  of  species  study  included  i t p r e s c r i b e s the  i n a community-wide c o n t e x t  Chapter  For  reported  commonly  studied (Fig.  study  of  species.  (only  which reduced  the  the  another  s p e c i e s of a community.  inferior  accommodation  one  repens  combining  communities.  s p e c i e s from  of b i o l o g i c a l further  of t h e  failure  competition usually  was  as a s e l e c t i o n a l p r o c e s s  competitive a b i l i t i e s  the  combining  but  a  (a, b or c ) .  study  for  from  ecological  nonetheless,  natural  mechanisms of  of  of  a s s o c i a t e d w i t h any  selection  found  increased  the  ability  i n the p r e s e n t  species combinations  in  of  on  pair  pastures,  force  in Trifolium  i n t h e component  exhibited  aged  of c o m p e t i t o r s  combining  pressures  of  given  It i s evident that  effects  was  a  different  (1979c) c o u l d be  evidence 11a)  of  important  3 c o n s e q u e n c e s of n a t u r a l  notion  instead  an  system.  and  & Harper  (Fig.  the  is  specialization  combination)  in  from  interpretation  specialization the b i o t i c  mixtures  species  reciprocal  permit  example,  of  competition  in this  study  does not  yields  neighbouring that  selection to  total  model  for  and  pasture  1 32  CHAPTER 5  CHANGES  IN THE NATURE OF COMPETITIVE RELATIONS  AMONGST THREE SPECIES IN DIFFERENT AGED PASTURES SUBSTITUTIVE REPLACEMENT SERIES MODEL  133  INTRODUCTION In the  a  diallel  performance  interaction  can  having  constant  de  1978).  Wit  of  It  1961)  the  of  has  of  mixtures  a  oh  d e p t h on the  the  legume  abilities  genotypes or  species.  proportions  of  Anthoxanthum  dependence w i t h &  de  The  (de Wit  a c l o s e balance 1960).  shown between T r i f o l i u m  niche  et a l .  1966) in  1973).  resources  in  independence o f  found 1960).  to  be  relative between  oats  in competitive  Lolium  sown i n  independent  In c o n t r a s t ,  Phleum p r a t e n s e  and  of  overlap  a d v a n t a g e of  Frequency-dependent repens  of  competitive  & Harper  limiting  of  competitive  o d o r a t u m and  effect  placement  interpretation  degree  peas was  in mixture  Wit  root  (Trenbath  the  1974b).  permits  and  s e r i e s with  the  models  (Trenbath  the  the  frequency^dependence or  competitive  these  competitive  (de Wit  differential  genus Avena  interactions  replacement  on  quantifying  how  on  on  density  experimental and  of  that  data  Based  e n v i r o n m e n t ; e.g.  and  type  maintaining  analyzing  Rhi z o b i urn  grass  competitive  Bergh  for  interactions (Hall  Information  using  series').  a l s o been u s e f u l f o r i d e n t i f y i n g  competitive  a  but  4),  of  r e l a t i o n s h i p s between p l a n t s  symbiotic  of  analysis  further  for i d e n t i f y i n g  i n f l u e n c e d by  soil  The  introduced  used  between  effect  species  used  (Chapter  reveal a particular  carried  'replacement  been  are  relationship  It  a  (1960,  has  presence  can  competition  of g e n o t y p e s . be  competitive  interactions  and  mixture  a range of p r o p o r t i o n s  have been w i d e l y  properties  the  1:1  however  (i.e.  criteria, which  in a  for studying  between a p a i r  interaction mixtures  design  showed  of  mixtures  frequency-  ability  (van  den  behaviour  has  been  perenne  which  is  •  :  1 34  interpreted species  De  1960,  Wit  de  a history  of  series  by  product  past  replacement  particular  site  and  -  sexual  the  in  an  repens -  as  t o one  effort  to  having  studies  another detect  pair  of  from t h e  of  The  the  pastures  age -  Holcus  field. presence  of  mentioned  series.  r a m e t s of in  2  In  single  immediate  T h i s a p p r o a c h was of  repens  individuals  replacement  growing  in are  studies  relations  cloned  the  Changes  species  of in a  in  study  non-identical  neighbours  i n the  species  perenne - T r i f o l i u m  i n the  uses  using  degree  pasture  competitive  study  evolved  investigated  pairs  only  studies the  perenne.  reproduction)  Laitinen  i s an  how  increasing  , Lolium  &  of  present  progression.  genetically  natural  is  collected  in  history  Joy  previous  This  Lolium  no  1978,  particular  with  present  genets c o l l e c t e d proximity  No  different,  used  to  replacement  differentiation  developmental  changes have  of  a  plants  lanatus  previously  contrast,  using  for.three  evolutionary  (products  in  (community).  Trifolium  Holcus  response  in  with  These  that niche  relations  investigated  for analyzing  in  behaviour  - i n Snaydon  competition.  t o a common  competitive  lanatus  species  populations  1981).  changes  study  belonging  of  s e r i e s a n a l y s i s have c o n s i d e r e d  overlap  present  relations  do  Harding evidence  two  g e n e r a l l y form more s t a b l e a s s o c i a t i o n s  than  circumstantial of  a s e n s i t i v e method  frequency-dependent  &  i n these  1961).  (Remison & Snaydon  Martin  differentiation  s t u d i e s have shown t h a t p o p u l a t i o n s  interaction  analysis  interaction  niche  model p r o v i d e s  Previous  demonstrated  niche  Wit  i n the c o m p e t i t i v e  selection.  1980,  some d e g r e e of  (Ennik  The changes  as  localized  taken biotic  135  specialization  of  e s t a b l i s h e d g e n o t y p e s and t o i n v e s t i g a t e t h e  developmental q u a l i t i e s ages.  In o r d e r  facilitate theory, will  of that  specialization  to s e t the stage  interpretation  a brief  review of  of  as the  f o r the experimental the  results  relevant  d e s i g n and  based  theoretical  community  on  accepted  considerations  follow.  THEORY If  mixtures  variety  of  the  species.  a)  an  the  Five  change  elapsed  type  a  of  basic  of  12a).  unchanged a f t e r proportion  The  ratio  of  i t indicates (assuming  that  f o r both  species,  :  the  intraspecific  same  individual,  all  In t h i s  considerations  capacity  i n a species  proportion  ratio  resources  below,  be  for  interpreted  an  Since  (i.e.  their  final  ratio for  individual  enough) provides  as  t h e two s p e c i e s  does  niches  will  frequency  t o the i n i t i a l  an  make t h e same broadly  has any ' a d v a n t a g e ' .  'advantage' its  remains  the  proportion  competition  species  to increase  relative  - a  between two  d e n s i t i e s are high  of  and t h a t  case n e i t h e r  display  t h e two s p e c i e s  interspecific  intensity  demands on e n v i r o n m e n t a l overlap).  can  the i n i t i a l  mixtures,  precisely  those  1977b):  all  an  of  at a  so p r o d u c e d  relationship  of growth t o g e t h e r .  e x a c t l y equals  ratio  figurative  interaction  proportion  a period  the  The p l o t  competitive  types  are planted  t i m e c a n be p l o t t e d a g a i n s t t h e  sensitive  12) (de W i t 1961, H a r p e r (Fig.  in  at the beginning.  diagram'- provides  detecting  (Fig.  after  of p r o p o r t i o n s  'ratio  two g e n o t y p e s o r s p e c i e s  proportions,  proportions ratio  of  refer in  proportion  the ratio.  to  In a  final  136  FIGURE a  12.  R a t i o diagrams  replacement  competitive  series  represents  equal  a l l final  text  for  experiment  relationship  line  the  ratios  description.  illustrating  between  5 possible  reflecting species  theoretical  outcomes  5 different  i and  j .  t y p e s of  The  c a s e where a l l i n i t i a l  ( i . e . the l i n e  of  from  'no a d v a n t a g e ' ) .  dashed ratios See  137  Initial  log  i/j  138  b)  (Fig.  12b).  Here,  at  a l l relative  is  interspecific,  more  in  the  If  initial by  once a g a i n  for  the  the  as  by  a  availability  of  distance 'no  frequency of  the  advantage'  relative (Fig.  that  the  that  increase  corresponding  proportion  capacity  ratio,  in their  from the  i is  reduce  the  which they  both  same  Accordingly,  regardless  the  of  perpendicular  theoretical the  use  by  to  on  the  line  of  s u p e r i o r i t y of  i's  ability.  Here,  gains  an  in a  is  shown,  advantage possessed  i n d i c a t e s the m a g n i t u d e of  12c).  as  j overlap  i s the  for j  competition  i.e.  j from a s u p p l y  line  in mixtures  competition  i s 1.0  final  i and  the  capacity  actual ratio  line  results  greater  to  this  the  intense  frequency;  in mixture.  competitive  j  of  Hence,  resources that  of  in  relatively  make demands, and relative  intraspecific  i n a)  available resources.  advantage  i ,  ratio  same amount,  an  most  slope  proportion  indicating  reflected  c)  whilst  i gains  The  f o r i i s independent  increase,  of  frequencies.  intense.  advantage  species  an  the  same p r i n c i p l e s  advantage  in  as  i n b)  mixtures  at  apply all  except  relative  frequencies. d)  (Fig.  12d).  This  result  indicates  competitive  relationship,  i.e.  is  on  proportion  dependent  minority  to  competition 'advantage' by  initial  component  consequently contrast  the  the  in  regression  b),  each  f r o m an f o r the  i t s experience  a  of  component  individual minority fewer  of  frequency-dependent  which s p e c i e s  mixture slope  a  ratio.  has  an  Generally,  i s a l w a y s a t an  is less  than  experiences i t s own  component  intraspecific  advantage  advantage  1.0. the  the  Here, most  species.  of a m i x t u r e individuals  in  intense  Hence,  the  i s incurred and  hence  1 39  less  stringent  implies  that  overlap; occurs 'no  resource  i and where  the  of  imposed  equal  majority  I2e).  component of  1.0.  of  a  the  Intraspecific  ratio line  of  identical  species,  the  of  resource  those  hardships  e l e m e n t s of  on  an  unstable  situation  example w i t h the  the  highest  i f each  more t h a n  niche  of  for  relative capacity component  one numbers  i n the  to reduce species  the  slope  i s greater  stronger  result  the  other  frequency  i t s own  effort is  in mixture  environment  (Harper by  one  with  of  and  a consequence  proportion  ratio.  resources  relative  for  1977b).  component  amongst c o h o r t s ,  availability  Such.a  e n v i r o n m e n t , as  entirely  initial  the  than  This  out-compete  impaired  the  advantage,  species.  interference  changes  instance, an  never  interference  component  frequency-  therefore exists.  species  mutual a l l e l o p a t h i c effectiveness  is  relative  i t impaired  at  regression  point  a  in this  always  could  encompasses a c o n c e r t e d  'advantage'  but  either  equilibrium  would o c c u r other  in  species  w h i c h has  indicates  competition  depending  other  to  f o r those  also  Hence,  species.  either  the  an  hardships  magnitude  is  that  b),  in  mixture  predicts  higher  f o r both  relationship  competition  another  because,  result  interspecific  Here,  theoretical ratio,  imposes  completely  equilibrium  proportion  competition  This  competitive  regardless  the  does n o t  An  i n t e r s e c t s the  traditionally  overlap.  dependent  of  species  niches.  density  are  plants, this  two  results  interspecific  (Fig.  and  the  initial  ratio  that  by  w h i c h do  than  this  intraspecific  limitation  by  regression  At  proportion  level  use  With  j have d i f f e r e n t  advantage'.  final  e)  competition.  on an of  Unlike to  frequency  the in  140  mixture, in  and  such  this  this  an  applies  interaction,  model p r e d i c t s ) ,  overlap  concurrently  was  competitive  i t would  enough  to  to both exclusion  imply that  make  components.  the  was  imminent  extent  impossible  any  If  of  (as  niche  escape  from  extinction.  METHODS Three d i f f e r e n t repens, perenne  Lolium  genet  perenne  -  , were c o l l e c t e d  as  each of the 3 d i f f e r e n t years). Chapter  Each  pair  2 to obtain  Plastic  pots  containing  a standard potting  mix  series  and  arrangement  •arrangement  ensured that  was  randomly when pots :  shown  as  received  three  whenever  the  average.  After  weeks  soil  diameter,  genet  in  pair  1, 5  surface  4 weeks, an  - L.  of  t i m e s and  in  40 in  type. deep) 25%  replacement  by  planting component ':  ramets  each  of the-  replacement  t h e p o t s were a r r a n g e d culture  organic The  was  1980)  fertilizer  pots  became d r y , a b o u t 'initial'  cm  The  week o f J u n e ,  9).  and  sand and  minority  Rhizobium  of l i q u i d and  13.  the  15  the  possible,  (first  from  type, a constant  in  Figure  as  tunnel.  were p l a n t e d  (2, 21  f o r each genet  Each p r o p o r t i o n  3 applications  10/5/5) ( a f t e r  Trifolium  p r o p a g a t e d as o u t l i n e d  planted  much  in a polyethylene  -  lanatus  of 50% p e a t , 25%  e a c h ramet  type.  replicated  the ramets  cm  p e r p o t were  surrounded, component  and  For each n a t u r a l  13 ramets  , and H^  of ramets 15  of  series  collected  -  density  majority  repens  lanatus  immediately adjacent neighbours  liter  were u s e d .  was  T_^  a population  (1.75  Holcus  aged p a s t u r e c o m m u n i t i e s  was  perlite  type  pairs,  were  added and a l l (N/P/K watered  3 times weekly  count of the t o t a l  on  number  141  FIGURE  13.  replacement s p e c i e s _i species j  Planting  arrangement  used  for  8 5  4  13 r a m e t s  i n the  series: 0 1 2 1 3 1 2 1 1  4 9  5 8  9 1 1 1 2 1 3 2 1 0  142  5/8  143  of  tillers  (for grasses)  (for  clover)  After  a further  'final'  per  tiller  and measure o f t h e t o t a l  p o t were t a k e n .  This  12 weeks, t h e e x p e r i m e n t  stolon  f o l l o w s Ennik was  length (1960).  terminated  c o u n t a n d s t o l o n measure p e r p o t were  and  a  recorded.  RESULTS AND DISCUSSION For ratios  each  were c a l c u l a t e d .  plotted  as r a t i o  regression Ratio in  replacement  interspecific  established the  alternative  component  species  of  parallel  rotation  (Fig.  The s m a l l e r  of  regardless  zero,  the  of i n i t i a l  components  final  relative  virtually  These  are  from  in  reflects  selection in  considered  below  ratio."diagrams f o r i n t h e -slope  a process  a  common of n i c h e  differentiated  requirements.  frequency  frequency,  • of  the p o s i t i o n of the  t h e more  to niche  one  from  species i n t e r a c t i o n s .  relative  avoid  with  competitive  of n a t u r a l  the slope,  respect  changes  the context of  of the r e g r e s s i o n with  15a),  1976).  accordance  changing  changes  slope  t h e two components w i t h  slope  two  2)  In  were  linear  for analyzing  r e l a t i o n s : • 1) Changes  A decreasing  differentiation.  a  for  consequence 15).  multiple  within  v a l u e s .are e x t r a c t e d  competitive  regression;  regression.  are  a  (Fig.  critical  interpreting  point  as  the data  diagrams produced  interpreted  models  proportion  (Fox & G u i r e  relations.  by a d i s c u s s i o n o f p a r t i c u l a r  Two  the  be  hypothetical produced  model  12), the r a t i o  may  relations  followed  a convenient  (Fig.  data  and f i n a l  14) f o l l o w i n g  a MIDAS program  competitive  theory  present  (Fig.  using  diagrams provide  initial  After log transformation,  diagrams  analysis  series,  which  is  With  constant  implies that the  another completely.  The  1 44  FIGURE  14.  point  represents  final  ratios  R a t i o diagrams the  involving  f o r the experimental  value total  number p e r s p e c i e s p e r p o t .  corresponding stolon  results.  Each  to the i n i t i a l  and  l e n g t h and/or  total  tiller  145  146  -2.00  0  2.00  4.00  1 48  FIGURE  15.  evolutionary  Ratio  diagrams  changes  species  ( i . and  natural  selection.  j)  in  competitive  during See  showing  text  different for  theoretical relations stages  description.  trends  for  between  two  ( 1 , 2 and  3) o f  149  150  performance the  of  other.  towards  each  is entirely  I t follows that a reverse  1.0 would s u g g e s t  because  of  interaction,  the f i n a l  relative  confused  that with  the f a c t  component  increasing  in  slope  beyond  increasing  mutual  components  (not  of  z in  the r e l a t i v e  both  b)  shown  and  to  be  competitive  above  using  resource  selection changes ability the  are  less.  niche  reflected  revealed amongst These  each the genet  species  each  are  species pair  model ( F i g . from x t o y t o  reflects  changes  c)  nor  and  3.  niche  c) i m p l i e s  that  Evolutionary  relative change  position  pair  analysis  studied,  to i i s  their  overlap.  and  In  i n b ) , i and j a r e  in  b)  2  in i t s parallel  competitive  i n the slope of (Fig. of  15d). variance  d i f f e r e n c e s i n s l o p e s and i n t e r c e p t s  p a i r s from d i f f e r e n t  comparisons  that  by a c o n c o m i t a n t  significant  two  i n the p o s i t i o n  1,  whereas  relationship  the  of j r e l a t i v e  stages  Neither  be  An  o f t h e two components.  t h e degree of n i c h e  r e g r e s s i o n and s h i f t For  for  has changed  shifts  15c),  dictate  supply,  to  of s e l e c t i o n f o r  between  ability  through  however  overlap  i n both  abilities  much  frequency.)  independent  (Fig.  the c o m p e t i t i v e  Considerations  requirements  of  of the e q u i l i b r i u m p o i n t  d e p e n d e n t model  where,  = 1.0, an a d v a n t a g e f o r  Parallel -  slope  very  (This i s not  effects  frequency  increasing  t h e same  frequency  independent  interference  in a  c)  convergence  1.0 i n d i c a t e s a p r o c e s s  or a displacement  i n a frequency  is  illustrated).  the regression  15b),  mixture  frequency. a slope  by t h e p r e s e n c e o f  of i n c r e a s i n g  f o r niche  relative  with  order  selection  depends on t h e i n i t i a l  a  unaffected  displayed  follows.  aged  pastures  in Figure  ( F i g . 14).  16 and a d i s c u s s i o n  151  FIGURE from  16. the  R a t i o diagrams three  superimposed variances  different  aged  on t h e same g r a p h . after  Probability significant regressions comparisons given  f o r immediately  are  from  differences (* s i g n i f i c a n t of  field  in  age:  age.  2 year;  (2,  genet  pairs  21 and 40 y r s )  F - t e s t s f o r the homogeneity  an  were  analysis  slopes  and  not of  A coefficient  of  rejected.  variance  intercepts  a t P<0.05) f o r a l l p o s s i b l e  f o r each m u l t i p l e l i n e a r  Pasture  pastures  log-transformation  levels  adjacent  of  for the  pairwise  of d e t e r m i n a t i o n i s  regression. 21 y e a r ;  40  year.  152  /  1  1  1  -2.00  Log  r——i  Log  g  0  1  2.00  (Trifolium/Holcus)  -2J 00  Log  1 0  '  2T0O  (Trifolium/Lolium)  (Holcus/Lolium)  1 4.00  (Initial)  '  4.00  (Initial)  (Initial)  153  a)  Holcus H.  lanatus lanatus  regression position  the  two  and  slope  of  unstable  behaviour year  old  pastures.  This  genet less  slope  but  do  shifts  p r o p o r t i o n a l l y to  pair two b)  differ  .  no  trend well  as  in  than  1.0)  this  i n the  i n the  40  Figure  a  shifts  year  repens  An  a  stable  two  The  older  21  year  regression  The  regression  all  relative  niche  for  in  at  p a i r than  d i f f e r e n c e i n amount of  the  I5d).  to  differ  higher  in  is illustrated  (intercept).  T\  indicates  a shift  divergence.  significantly  favour  as  decreasing  p a i r s from t h e  niche  postion  of  in  relative  competitive  the  overlap  21  year  between  the  r e l a t i o n s h i p between t h e s e  two  components. Lolium  species of  p e r e n n e - Tr i f o l i u m  the  genet  regression This  pairs  pasture  competitive  p o s i t i o n of  an  example p a i r s do  regression favour  not  two  T\  slope  16b).  <  1.0)  pairs  niche  from  overlap  older pastures  but  from  niche  model  d  in Figure  significantly As  differ  with  p e r e n n e and  15;  T^  i.e.  the  and  repens s h i f t s  repens at a l l r e l a t i v e  the  older.,  between youngest  convergence. demonstrating 21  in regression  lanatus  slope  two  i s greater  than  of  the  the  r e g r e s s i o n a l s o changes, again  in position. for  in  (Fig.  a s e l e c t i o n process  the  of  (with  higher,  i n d i c a t e s that  suggesting  differ  is  from t h e  The  year  the  repens  i s frequency-dependent  pastures.  to  1.0)  in  This  In a l l c a s e s  do  not  for T r ifolium  but  and  i s i n t e r p r e t e d as  p a i r s do  ability  (i.e.  16a).  a  d i a g r a m as  pair  than  (Fig.  showed  (slope greater  year  frequencies  ratio  regression  (slope  40  repens  i n the  the  behaviour  and  - T r i f o l i u m repens  T\  year  and  40  slope  but  repens,  the  proportionally  frequencies.  T\  repens  1 54  evidently perenne than  has  a higher  for  i n the  c) H o l c u s  niche  year  pasture.  lanatus two  independent indicates have  common 21  These  widely  competitive  requirements  perenne  overlapping change a s  there  i s no  H_;_  all  diminishes  with  increasing pasture  approaches  the  This  different  pastures  of  performance Drosophila.  differential  is  in Antonovics performance  'no  the  to  which  two  is less  of  advantage this  40  of  difference in  year  in  advantage regression  advantage' 2 year  and  is  regression.  lanatus  in plants  competitive  as  of c o m p e t i t i o n ,  result  studies  expected, resource  van  output  in  Deldon  of  on  have  changes  is  from  the older  interaction  by  one  or  Barker  (Sokal  shown et  1952,  Chen that  al  .  as  increased  components  (Moore 1973,  involve  changes, of  both  the  competitive which  direction  O t h e r work has unchanged  most of  the mixture  1970,  in  demonstrated  the  utilization  1978)) . remains  degree  15).  studies  1967,  perenne  this  an  H^  experimental' a  L_;_  This  divergence.'': A n t o n o v i c s "'( 1 978 ) r e v i e w s  reproductive  Antonovics  of  expectation  Several  theoretically  greater  of  traditional  between•. spec i e s findings  extent  (model b i n F i g u r e  The  the  i n i n t e r c e p t from t h e  the  competitor  and  but  age;  line  pasture  pairs.  and  has  frequencies  theoretical  i n d i c a t e s that  superior  relative  L.  frequency-  significant  lanatus  at  year  a  all  lanatus  requirements  mixtures  significantly  in  niche  regression slopes.  40  largely  H^  against  16c).  relationship  in a l l 3 pastures,  ability  i n the  (Fig.  demonstrate  competitive  that  three  - Lolium species  o v e r l a p does not the  relative  and  Seaton  1973  &  (cited  competitive 1970,  Hedrick  1 55  1973,  Sulzbach  1970,  Ford  1980), or  i n some c a s e s  1972).  Variable  unpredictability  associated with  even  results  of  genetic  variance  or c o n s t r a i n t s on  such  f a c t o r s as  negative  correlation  depression  (Antonovics  however may  also result  i f selection  different  ways  the  contradict of  an  'traditional  erroneous Studies  of  the  the  competitive  change  present  present  i n t e r a c t ing ( f o r the  competitive  environment?  interpretation  nature,  not  were  Figure an  be  a  or  of  results  consequence  experiments  individuals  and  15). of  niche  and  relations  Unlike  genet how  species)  in  lengths  of  reported  not  meet  T\  divergence  are  w h i c h has  repens as  this  respond present  occurred  in  Individuals of  identical  interacting  offered may  does  the  traditional  selection  that  communities  in  w h i c h were a c t u a l l y  i s the  pairs  time to  here are  are  previous  q u e s t i o n : What  Furthermore,  interpretations  lanatus  demonstrations  laboratory conditions.  ways i n w h i c h n a t u r a l  hypothesis  variety  and  involves selection  R e s u l t s w h i c h do  found  different  i n the  the  nature  different  under a r t i f i c i a l  to those  field.  by  results  when  competitive  same two  to  study,  in  in  have had  the  to  caused  a  above  between e s t a b l i s h e d  populations  genotype  simply  investigation.  whose  interact  o r due  Variable in  to  components  arises  t o the  work a d d r e s s e s  relationship  relationship differ  which  selection  operates  may  due  designs,  1978).  expectation'  of  to the  actually  the  be  among f i t n e s s  which  (Futuyma  expectation.  progression  studies,  query  f o r p l a n t s comparable  unknown p r i o r  are  and  may  experimental  lack  inbreeding  declines  in  expectation  which  reflect  operate  (i.e.  show e v i d e n c e  traditionally  supporting  expected,  but  1 56  L.  perenne  and  repens  and  a l l 3 species pairs  has  changed  the  non-identical Results  f o r the  two  perenne  ,  balanced  relative  a  suggest  resource The  perenne  t o changes grasses  which they  repens  suggests  between t h e s e environment with  two  two  species  in Chapter &  between two  3.  Levins  suppressing  a third  Convergence  selection  is  They  argue t h a t a competitor  in competition  evolving and  to  (Wilson  and  ,  initially  i n a more  species  for may  to  some form  be  related  Vallis  1978).  stable  association  pasture  animal  age  i s more  competitor  (Sheppard  able to  reducing  seed  common . t o  the  kinds  1975).  in  increase i t s relative  production  result  effective  Agren  competitor  two  times.  competitor seedling fitness  respect to flowering of  of  for  flowering  inferior  was  competition,  i s s u p e r i o r to  but  to a  1942,  in their  which  of  through  associated, with  differ  L.  grass  together  aggressive  pollinators, be  for  of  t h a t c o n v e r g e n c e may  activity  A,  L.  the  increasing  increased niche overlap with  thereby  overlap.  (1980) p r o p o s e a model of n i c h e c o n v e r g e n c e  s p e c i e s which  competition  two  T h i s may  increasing  also  plant  B  solely  f o r c e s a s s o c i a t e d with mimicry  & Fagerstrom  of n i c h e  resulted  respect  more  species, a  make demands.  (1967) s u g g e s t  species i f their  the  of  the p o s s i b i l i t y  with  In  selection  n i c h e c o n v e r g e n c e between  the c l o v e r  these  that  lanatus  species.  of  convergence  }U_  made a v a i l a b l e  between  MacArthur  has  both  and  for a strong trend  both.  16c),  suggesting  Evidence  in  (Fig.  that selection  on  association  presented  i n the e x t e n t  result  nitrogen-rich  suggesting  competitive a b i l i t i e s  unusual T\_  for niche  c o m p e t i t i v e power between t h e  supply  beneficence  close  show e v i d e n c e  relative event  show e v i d e n c e  B.  by  time, Such  157  considerations occupation  with  interaction, competitive evolutionary neighbours.  draw  'divergence' and  focus  as  adjustment In  this  between  in  chapter.  away a  of vein,  from  traditional  consequence  instead  power and c o n c o m i t a n t  interaction the next  attention  on  the  beneficial  species  to  p e r e n n e and T\_  role  competitive of  relative  interactions  their  a finer-scale  of  pre-  i n the  environment  investigation  repens w i l l  be  of  of t h e  presented  158  CHAPTER 6  BIOTIC SPECIALIZATION AT THE GENOTYPE L E V E L : RECIPROCAL PHYTOMETER TRANSPLANTS AMONGST FOUR NATURAL NEIGHBOURING GENET' PAIRS OF LOLIUM PERENNE AND TRIFOLIUM REPENS  159  INTRODUCTION  Population natural  is  selection.  individuals of  ecology  all  leave  affected  and  study  related  processes 1978)  interaction  of  and  has  the  most e x c i t i n g f i e l d s has  had  ecology.  these  in  the  consequences at  a relevant a  (Antonovics  1976a, H a r p e r  r e g a r d e d as  local  not  the  different  9,  Chapter  and  set  1977b,  species,  the  not  1967,  need  the  Harper more ecology  provides  one  of  of  the  community with  the  species  different  needs  to  diversity  events  community  community  and  regard and  to  components  in  attend evolution  be  which,  p a r t i c u l a r genotypes which  as  (Antonovics  literature  the  of  Such  These components may  the  of  the  for  population two  community  but  of  of  biotic (Fig.  3).  community-based  population  why  ecologists  1982).  itinerary  Most d e m o n s t r a t i o n s of are  and  c o n g l o m e r a t e of e v o l v i n g  neighbours d i c t a t e  interaction  of  neighbourhoods w i t h i n  particular  consequently  level  in recent  u n i t of  properties  dissatisfied  with  those  been a i r e d  as  the  categorization  than  some  i n modern b i o l o g y .  become  formulated  mind  the  of  are  Since  and  that  The  the  genetics  of d e v e l o p m e n t  were  community  to  c o n v e r g e n c e of  have  I n t e r e s t has  g e n o t y p e as  view  recent  categories  1976a).  attention  'typological'  philosophies  the  the  Ecologists  taxonomists'  how  resultant patterns.  and  thinking  that  asks  study  dictates  organization  population  the  than o t h e r s .  ecology  called  between  extent  variation  biological  has  stressed  large  more d e s c e n d e n t s of  (1967,  a  Genetic  levels the  to  adaptive  genetic  s t u d i e s ; they concern  from a d i v e r s e  species  v a r i a t i o n in  plants  a  s i n g l e component  assemblage.  Investigations  160  have  shown  different even  wide (Aston al  Very  evolution To  at  the  the  some  degree  therefore  and  will  quality  of  'con-trophic',  in  plant  constituent  i n s p i t e of 1966,  salinity  been  community  the  the  to a l a r g e  species  is  the  s t r u c t u r e and extent  work has  be  Most  studies  again  been  concerned  within  a single species.  as  habitats  in  soils.  r o l e of  such  structure  and  amongst  It  is  by  an  on  examination  shown  biotic  with For  the  . of  of  such v a r i a t i o n  the  of of  the  another. in  however,  1969)  a  reciprocal  t o one  behaviour  (Watson  of  this,  biology  interactions  example,  quantity  properties  micro-evolution  differential  plant  interpretation  i n d i v i d u a l s respond  have to  based  which  of  the  spite study  group  reasonable  evolution  In  been done t o  study  i n which n e i g h b o u r i n g  in response  level  determined  community-level  requires  which  such  is essentially a  presumed.  . interactions..  vegetation  Raven  (Antonovics  1972)  v a r i a t i o n r e l a t e d t o the  Any  &  environments  metals  same t r o p h i c  interaction  little  diluting  forces,  of  pool  level.  on  genetic  the  the  to  e c o l o g i s t a community  that  gene  in maritime  heavy  given  enough,  Ehrlich  (Snaydon & D a v i e s  has  strong  single  studies,  l e v e l s of  manner  populations  a  v a r i a t i o n in considerations  communities.  descriptive  of  fertilizers  biotic  surprisingly  most  degree  species  of  and  is  abiotic selective  1966), or  to expect  communities  In  stringent  plant  co-occurring  arise within  distances  attention  genetic  selection  ( J a i n & Bradshaw  the  or  little  adaptive  of  in  1971)  can  1972).  & Bradshaw .  'types'  flow  concern  ranges  disruptive  short  Bradshaw  discussed  et  very  from gene  1969,  if  adaptive  over  effect  that  plant have  (responses) found  that  161  populations  of  Potentilla  of M o l i n i a - and in  respect  transplant  size  material  of  characters  and  garden.  as  seed  Linhart  portion  of  a  distance  of  only  the  single  t o be  central  near 1978)  characters,  five  r e l a t e d to region  competition Antonovics  to  found  Plantago  compared  progeny  growing  meters.  that  with  Watson respect  lanceolata collected  h a b i t a t s were g e n e t i c a l l y d i f f e r e n t i a t e d vegetation  height  from  which  they  Trifolium  repens  response  exerted  different  permanent  in  species  ecotypes  'Darwinian'  f o r c e s of  discussed  above  further,  on  two  an  central over  competition  (1974,  a  in  cited  several  a  in  growth  respect  Turkington in  microto  &  the  Harper  population  selection  in localized  the  at  the level  g e n o t y p e as elaborate  (i.e.  interacting  populations  natural  A community  more  concomitant  within  selection  have d e m o n s t r a t e d  specialization  populations. focuses  the  from d i f f e r e n t  to d i f f e r e n t grass  in  of  pressures  regions  of  a  pasture.  Biotic  biotic  of  both  inter-specific  to  came.  for micro-evolution  as  differentiation  with  (1979c) f o u n d e v i d e n c e  by  The  intense  periphery.  different  peregrina  intra-specific  more  areas  differentiation in  of V e r o n i c a  intense  versus the  when  individuals  population  two  were  (1974) d i s c o v e r e d  between p e r i p h e r a l i n d i v i d u a l s and  appeared  in neighbouring  A g r o s t i s - dominated g r a s s l a n d ,  to  experimental  e r e c t a growing  in  species context  a relevant  questions:  a  evidence  in  nature.  of  truly  Studies  fine-scale genetically-based  reciprocal) biotic  species  are  level  within  single  for micro-evolution u n i t of Does  diversity  there  exist  specialization  community?  Does  that  raises local  i n each  of  specialization  162  occur  at  the genotype  differentially different  level?  within  neighbouring  a  single  reciprocal  Allard  & Adams (1969) showed  yielded lines  higher that  difference and  had  no  than  history  in yields (1980)  several  pratense  that  generations  chosen c u l t i v a r s selection  of  adjust  specialization combinations? interacting  had  than  by  behaviour  the  Does  it  interaction  of  evolutionary These that  involve What  reciprocal  a  shifts  and  (from  existing  the  evolutionary  average  contrast,  what  such  in  does biotic  grass-clover changes  a possible  c o n c u r r e n t l y with in  for  randomly  way(s)  in  in and  mixture other  no Joy  advantage  in  both  beneficial  the n i t r o g e n - r i c h  environment the  negative  establishing  the  interactions? a view of  responses  received l i t t l e in  two  plants  might  coevolution  ignored  of  in  level  in genetic constitutions  virtually  on  had  stands.  yield  In  have  underscore  phenomenon t h a t has  thinking  role  that  In  pure  evolutionary  to neighbour  questions on  mixture  genotype  is  of Phleum p r a t e n s e  species.  competition,  response  i s centered  mutual  legume),  to  higher  of  and  g e n e r a l l y showed  cultivated  a  the  at  a  been  same  populations?  the  compared  the  i n t e r a c t i o n . of commensalism provided  interaction  to  genotypes?  barley  stands.  in strains  in  species,  of  occur  response  many g e n e r a t i o n s  pure  discovered  in  neighbouring  lines  over  of  in mixture  single  i n the  in  ( o v e r p u r e components)  Trifolium  population  that  in mixture  in mixture  Laitinen  mixture  specialization  together  does m i c r o - e v o l u t i o n  g e n o t y p e s of a  there  persisted  - i.e.  of to of  the p l a n t  plant  pasture  competitors  competition interacting  attention  community  involving species  in evolutionary  ecology  (Antonovics  1 63  1976b,  Snaydon  biotic  1978).  interactions  incomplete  and  without  the  actually the of  the  'combining  genotypes coexist  in  (Harper  ability  is  species,  In  work  differentiation  responses  interacting Previous  niches  the  ensuing the  genotype  do  1942,  Ahlgren  response  et a l  .  been shown t o v a r y  not  or  or  assumes  to  that of  'ecological' mechanistic ability,  broadly imply  by  overlap  that  interaction  contest  all  does  not  of n i c h e  in  lead to  the  would  other.  level  local  population  i s studied in respect  genotype  p e r e n n e and  neighbours Trifolium  Different  i n response  repens  to  from  repens .  g e n o t y p e s of a  to T r i f o l i u m  1945).  another,  requisites  s t u d i e s have shown t h a t d i f f e r e n t in their  species  to t h i s  here,  between n a t u r a l of L o l i u m  presence  two  necessarily  presented  populations  species d i f f e r  also  at  i n the  term,  rather that  by  reflects  combining  e l e m e n t s or  f o r w h i c h an  e x c l u s i o n of one  the  reciprocal  but  which  r e q u i r e s some measure  alludes  not  neighbours  ability  one  prefixed  their  t o any  of  is  responses  individuals  tacitly  ecological  does  precluded,  in respect  competitive  Garber  This  of  with  theory  1964)  c o e x i s t because  interaction  either  The  (Harper  4)  capacity  Traditional  species with  1977b).  place  the  patterns  of n a t u r a l  i s compatible  s y s t e m s of c o m p e t i t i o n  interpretation;  have  3).  p r o p e r t i e s of  reciprocal  Combining  interaction  differentiation.  definition,  of  (Chapter  community.  to  continue  (Chapter  combining  take  ability'  i t refers  to  coexistence niche  investigation  i n the  the  micro-evolutionary  t o w h i c h e a c h component  other;  about  R e c i p r o c a l responses  interact  extent  ensuing  an  between n e i g h b o u r s . defines  Information  grass  (Myers  &  c l o n e s of c l o v e r  to a p a r t i c u l a r  grass  1 64  (Turkington  et a l  repens  the  in  increasing pastures  (Fig.  two  species  designed  Chapter  is  to t e s t  the n a t u r e  whether  of  as  oldest  (1939) p a s t u r e d i f f e r e d of e a c h  pairs  series  between  ability  5).  here.  The  The  different  these  of  T.  extent  ability  of L o l i u m and  in their  in older  replacement  the combining  from  T.  p r o g r e s s i o n of perenne  competitive  investigated  taken  presence  a  with Lolium  (Chapter  samples  neighbouring  showed  1979c).  f o r niche convergence  pastures  further  Harper  The  increased r e l a t i v e  and  &  site  3).  evidence  older  specialization  study  of a s s o c i a t i o n  8,  showed  in  Turkington  present  s p e c i e s and  repens  1979,  stability  experiment two  .  of  of  these  experiment  was  Trifolium  clones  localities  i n the  ability  t o grow  in  the  other.  METHODS  Experimental Four repens  different  genet  pairs  of L o l i u m p e r e n n e and  neighbours  pasture  percentage  from  where  cover  four widely there  was  frequencies.  The  separate abundant  pairs  r o o t m a t e r i a l ) and  propagated  separately  under  by  s e p a r a t i o n of t i l l e r s  periodic  followed  by  (26.5  standard  replanting cm  as  mix  of  overlap  each  50%  30.5  or c u t t i n g  cm  peat,  genet  of  in  the their  as  'type'  stolon clone.  deep) were u s e d 25%  in  as  two was  c o n d i t i o n s f o r 4 months  r a m e t s of t h e o r i g i n a l  top diameter;  potting  glasshouse  locations  were c o l l e c t e d  whole p l a n t s ( i n c l u d i n g  pots  Trifolium  ( d e s i g n a t e d L 1 - T 1 , L2-T2, L 3 - T 3 , L4.-T4) were c o l l e c t e d  physical 1939  Design  sand  and  25%  pieces Plastic  containing a perlite  with  165  added half in  nutrients the  specified  recommended  abundance.  Each  seeds  of D a c t y l i s  as an  outer buffer  preventing pot  and  appropriate  rate pot  (Fig.  zone and stolons  zone was  zone by a p l a s t i c  TABLE  15.  were sown a t a h i g h d e n s i t y  to serve  'contain'  from  growing  and  potting  mix).  83 109 43  (18-6-12)  g dolomite  out  adjacent pots  study  over  from  plants  by  t h e edge of  the  interacting.  c o u l d be m o n i t o r e d  border  composition between  Trifolium  g osmocote  the  was  kept  s e p a r a t e d below g r o u n d  experiment  perenne  cm  The  by v a r y i n g  clipped. from  The  the  central  used  f o r the  'sleeve'.  Nutrient  competition  a 2.5  available i n which  height to achieve t h i s  border  had  t h e y were not  at  border  to  l e a v e s from  N u t r i e n t s were added  that  17)  the h e i g h t at which the D a c t y l i s Dactylis  15.  to ensure  glomerata  clover  keeping  in Table  of  potting  different  repens  .  mix  genets  of  (Values are per  (9 month slow  35  Lolium liters  of  release)  lime  g gypsum  4.5  g fretted  trace  elements  (boron,  copper,  iron,  manganese^  border,  8 ramets of  molybdenum, z i n c )  Three each  grass  'type'  in  weeks  after  the D a c t y l i s  ' t y p e ' were p l a n t e d w i t h all  possible  arrangement  had  4  surrounded  by  buffer  (Fig.  zone  sowing  the  paired  central  The  ramets 4  ramets  of  combinations.  phytometers  remaining 17).  8  central  (2 which  of  each  clover  The  planting  each  species)  s e r v e d as an  phytometers  inner  therefore  1 66  FIGURE  17.  genet  combinations  (T). The  Planting  arrangement of  f o r pots  L o l i u m perenne  containing  (L) and T r i f o l i u m  The o u t e r 2.5 cm b o r d e r was sown w i t h D a c t y l i s cummulative  followed recorded  in  yield the  separately.  of the 4 denoted  experiment,  each  different repens  glomerata.  central  phytometers  species  component  was being  167  168  experienced  the  each  phytometer  central  other  species.  ' t y p e ' was randomly  was  weekly  on  the s o i l  simulate  every  border)  grazing.  were  The  collected  each  to species.  the  the t o t a l  above  the  two  were  4  time,  the  arranged  The p o t s  were  2 to 3 times  weeks  the  plants  to approximately from  dried  parts  of  and  weighed  after  planting  the  4  central  and w e i g h e d and t h e w e i g h t s  added  to  3  the 4 c e n t r a l  T w e l v e months  ground  were h a r v e s t e d , d r i e d  components  to  of  were  g l a s s house.  3  -i.e.  ' t y p e ' and g r a s s  pots  clippings  according  phytometers  the  were c l i p p e d  separately ramets,  clover  s u r f a c e became d r y , a b o u t  Once  the D a c t y l i s  phytometers  controlled  environment  by 4 p h y t o m e t e r s  of  t h r e e t i m e s and  average.  (including  surrounded  combination  i n a temperature whenever  to  representative biotic  Each  replicated  watered  cm  most  the r e s p e c t i v e  of  cumulative  totals. Data  Analysis An  genet  index  of  'combining  combination  ability'  as f o l l o w s  was c a l c u l a t e d  (see Chapter  f o r each  L-T  4):  CA = y/Y' where Y i s t h e o b s e r v e d component  and  component. of  Y'  relative  value.  contribution reflects the  combination,  p e r e n n e assumed  compared amongst  of  their  same measure  (per pot) f o r the  i s the observed  (In each  Y and  value  genet It the  value T\_  two  capacity  repens  assumed  a  components to p e r s i s t  has  measure o f t h e to  the  Data  were  value  index i s strictly relative  combined  in interaction  for other combinations.  yielding  the  The CA  and t h e r e f o r e  that  yielding  f o r the higher  the v a l u e of Y ' ) .  combinations assumes  lower  yield  relative  to  tabulated in  169  4 rows and 4 c o l u m n s as i n T a b l e yields,  total  yields  t h e s e v a r i a b l e s was analysis  where, yield, the the  using  y. .,  column  6 • • d-  =  a value yield  + c• +  or CA  of  of  a  the  position  0 if i ^ j(i.e.  index);  linear  (Table  Each of  regression  16):  +e--i  i i s t h e row number; y  ( i . e . component  row  of the o b s e r v a t i o n  variable  i ) ;6  number;  j  is  i s t h e mean o f effect);  of the  due  r  i  is  observation  t o the e f f e c t  for  'diagonal  due  to the e f f e c t  has a v a l u e  of  incorporate a diagonal diagonal  total  i ) ; c.. i s a v a r i a b l e f o r 'column  observation  in a principle  clover  indexes.  1 ( a s s u m i n g no d i a g o n a l  effect'  is  i  yields,  f o r the observed v a r i a b l e ( i . e .  1, column  effect  j); d  diagonal  is  u'+r.  ( i . e . component  component  multiple  number; k i s t h e r e p l i c a t e  the  effect'  to a  v..,  i n row  to  combining a b i l i t y  subjected  a v a r i a b l e f o r 'row due  grass  model  component  cell  for  the f o l l o w i n g l i n e a r  is  column  and  16  cell);  e  effect' of  (i.e.  principal  1 i f i = j and a v a l u e  effect  ijk  i f the  is  a  of  of  observation  random  normal  variate. The o v e r a l l Ho: (i.e.  column or  had t h e n u l l  hypothesis:  r2 = r 3 = r4 = c2 = c3 = c4 = d1 no row,  primarily effects  test  column  to detect in  the  effects.  column  or d i a g o n a l  whether  data,  there  allowing  = d2 = d3 .= d4 = 0  effects). were any also  significant  an o v e r a l l  test  was  aimed  diagonal  f o r any p o s s i b l e row  Where t h e a n a l y s i s r e v e a l e d  effects,  A n a l y s i s was  no s i g n i f i c a n t  generated with  or row  the n u l l  hypothesis: Ho: (i.e.  assuming  row  d1 and  = d2 = d3 = d4 = 0 column  effects  are  zero).  For  170  TABLE text  16.  T a b u l a r model u s e d  i n the a n a l y s i s  of  variance  (see  for description).  CLOVER TYPE Tl  TYPE  LI  L2  y  V  +  y + c  l  + d  r  y  C  2  +  d  v  y  +  r  3  y  +  r  +  r  2  +  3  C  11  3  3  +  C  y + r, + o  2 C  L4  C  +  C  y + r  2+  c  4  3  +  d  y + r  3+  c  4  3 y + r  y  +  r  4  y  +  r  4  2  CO CO  L3  +  2  y + r + z  2  T4  T3  T2  4  +  C  2  V  +  T  A *  C  3 C  4  4  °4  +  171  individual  diagonal  effects  the  following null  hypotheses  were  tested: Hoi:  d1  = 0,  Ho2:  d2  D a t a were a n a l y z e d 1976).  F-tests  rejected  and  for  = 0,  Ho3:  using the  a  d3  = 0,  MIDAS  d4  program  homogeneity  a l l analyses  Ho4:  of  were p e r f o r m e d  =  0.  (Fox  &  variances  on  Guire  were  untransformed  not  data.  RESULTS The Lolium 18  results  of d r y  in a l l genet-pair  respectively.  diagonal pair  weight  effects  type  Lx-Tx  combination natural  combinations  The was  null  (Fig. Lx  neighbouring  are  in both  18), or Tx  the  yield  was  combination;  for  ; yield  combination •T1  and  pattern 19);  and  significant  L3-T3 .' ( F i g . f o r 'total  the  L1-T1  lowest  row,  For of T  and  17  and  column  each  genet-  (clover) in highest  and  any  in  L4-T4  the  showed true  i n the n a t u r a l n e i g h b o u r i n g  genet  diagonal effects  (L p l u s T) y i e l d s  converse  or  was  18 ) . • T h e r e was  combination  no  L3-T3 The  Trifolium in Tables  generally  (P<0.05) d i a g o n a l e f f e c t s . was  of  cases.  significant Lolium  for  shown  hypothesis  rejected  involving  production  were found  for  l i t t l e - neighbour  of t h e  showed t h e o n l y  L1-  specif ic  combinations  (Table  significant  diagonal  specialization  between  ef f e c t . The Lolium  fine and  comparison  scale  Trifolium of c o m b i n i n g  This  simultaneously  for  T  and  combining  nature  T  biotic  genotypes ability  is  was  L.  further  indices  i n c o r p o r a t e s the  for  ability  of  (Fig.  evident 19 and  from  Table  neighbour-specificity  For  each  genet  higher  than  in  any  pair  type  other  a  20). of  Lx-Tx,  L the  combination  172  TABLE  17.  Yields  each c o l l e c t e d perenne all of  combinations  types of  a  natural  four s i t e s  of c l o v e r  types  with  and g r a s s  their  3 replicates;  the p r o b a b i l i t y  levels  column e f f e c t s ;  type;  natural  repens ( T x ) ,  genet  i n t h e 1939 p a s t u r e  type  with  neighbouring  a r e shown on t h e p r i n c i p a l  significance and  with,  ( L x ) from  the c l o v e r  (g) o f p h y t o m e t e r s o f T r i f o l i u m  of  Lolium  and p l a n t e d i n  the performances  neighbouring  grass  d i a g o n a l ; a l l v a l u e s a r e means levels  given are  the  attained  f o r the four d i a g o n a l elements assuming f o r the o v e r a l l  test,  P=0.0041.  grass associate  L1  L2  L3  L4  T1  3 .32 ,  2..25  1 .00 ,  0...94  clover  T2  2,.95  3,.25  1 .73 .  0..99  producer  T3  1 .68 ,  1 .43 ,  3 .38  1 .39 .  T4  2.. 1 5  1 .07 ,  1 .84  2..97  P1=0.2102, P2=0.1301, P3=0.0074,  P4=0.0092  row  1 73  TABLE  18.  each  collected  repens  Yields  (Tx) from  (g) o f p h y t o m e t e r s with  all  combinations  of c l o v e r  of  the grass  are  shown on t h e p r i n c i p a l  replicates;  the  significance  levels  and  with  type  their  and g r a s s  perenne  genet  i n t h e 1939 p a s t u r e type;  and  of  (Lx),  Trifolium  planted  natural neighbouring  levels  given  clover  are  the  test,  P=0.0126.  clover associate  T1  T2  T3  LI  6..45  15.,22  grass  L2  7.. 1 1  .9..59  10., 1 3  5..21  producer  L3  1 1 .38 ,  6,,53  5.,63  8,.94  L4  12,.71  7,.92  10.,71  7,.44  •23. ,01  P1=0.0086, P2=0.2420, P3=0.0430,  T4  8..03  P4=0.6630  types of  3  attained  f o r the four d i a g o n a l elements assuming f o r the o v e r a l l  in  the performances  d i a g o n a l ; a l l v a l u e s a r e means  probability  column e f f e c t s ;  Lolium  a natural neighbouring  four s i t e s  types  of  row  174  TABLE perenne  19.  Total  (L)  and  neighbouring  combined Trifolium  pairs  from  i n a l l combinations  values  for  the  natural  diagonal;  (g) o f p h y t o m e t e r s o f L o l i u m  repens four  planted  principal  yields  (T)  sites  of grass  collected  in  neighbouring  levels  given  pairs  for  the four  diagonal  for  the o v e r a l l  test,  are the a t t a i n e d  elements assuming  3  T1  T2  replicates;  significance  row and column  type  T3  T4  10.18  L1  9.77  18.20  24.69  grass  L2  9.69  12.84'  11.55  type  L3  12.39  7.93  L4  13.65  8.92  9.01 12.10  6.29 10.78 10.41  P1=0.0125, P2=0.1346, P3=0.1133,  type;  the  a r e shown on t h e  P=0.0098.  clover  natural  t h e 1939 p a s t u r e and  t y p e and c l o v e r  a l l v a l u e s a r e means o f  probability  as  P4=0.3307  the  levels effects;  175  TABLE  20.  perenne  Combining  (L)  and  neighbouring  ability  Trifolium  pairs  from  i n a l l combinations  values  for  principal  the  effects;  natural  f o r genet p a i r s  (T)  sites  of g r a s s  collected  in  type  neighbouring  levels  four  given  diagonal  f o r the o v e r a l l  of L o l i u m  as  and c l o v e r pairs 3  type;  test,  assuming  no  replicates;  row  P=0.0014.  clover  type  T1  T2  T3  L1  0.52  0.19  0.1 1 ' 0.27.  grass  L2  0.35  0.36  0.24  0.21  type  L3  0.09  0.30  0.49  0.21  L4  0.12  0.17  0.13  0.48  P1=0.0045, P2=0.1287, P3=0.0088,  the  a r e shown on t h e  are the a t t a i n e d s i g n i f i c a n c e  elements  natural  t h e 1939 p a s t u r e and  d i a g o n a l ; a l l v a l u e s a r e means o f  probability for  repens four  planted  the  indexes  T4  P4=0.0121  or  the  levels column  176  FIGURE  18.  Trifolium  combination  T1, of  of phytometers  repens  combinations  compared  Yields  (T)  of genet with  to y i e l d s  when  types.  natural  of  Lolium  grown  together  G r a p h s show neighbours  the  i n a l l other combinations  replicates.  (L)  in  when  bar  involving  and  different  yields  (stippled  B) L2 o r T2, C) L3 o r T3, D) L4 o r T 4 . 3  perenne  in  graphs)  A) L1  or  A l l v a l u e s a r e means  1—•  1—•  o  ro o  -F  I ,.  l_.  o  oo  o  o  1  •  •  o  L YIELD (g)  T YIELD (g) t d  L YIELD (g) -F  NO  o  O  •  1  •  1  o  _1_  a-  -F  O  O  o  O  |  _1_  tO O  1  r-»  O  CO  cn  -F  ro  o  o  o  o  o  —1—  1  1  O  L21  -F  to o  L2"  to,  •^ • • • *• * *• * • "• * • • • *  04  o  O  -  I I  •v  * • * • *• * • * " • * • ** * • • * • *» * *  to  1  _L  tO |  II o  3>  EX  —l  r  1— 1  tO  T YIELD (g)  • • •• • ** * • • *» * * • —1 • * •* •  ro  • * *  •• • • •• • * • •  • • • •  » •  II O  * • * •  o  -a II o  Tl II O  o o  O to  00  cn  I to  -  — i» i i N3  —i' 1  ro  L3 —i NO  -F H  to  T YIELD (g)  L YIELD (g) t—' •fr  t—•  *—*  O  00  cn  -P  K>  O  O  o  o  o  o  o  _L_  1  1  1  1  1  K3  o  o  1  1  o  1  L YIELD Cg)  OJ  -P  to  o  o  o  o  1  o  J_  T YIELD Cg)  00 • o  cn • o  o  ...L_  1  —J  -P  •  • o  rO  •  1  _JL_  o  1  r  -p  r—»  •  o  O  •  o  -  —1  H  ro  r~  -F  1 1  -F  ~V  "0  II  O  cn cn  II o  -F  —I -F  o  r~  K> —1 -P  o o  O  S3  o  -P  II O  H  H  I  1  1 1  •  VM •  N3  H -P  r~  TJ II O  V  -P  o o  -p  00  1 79  FIGURE  19.  different  genet  T r i folium  repens  combinations compared or  Combining  of  type  ability  indexes  combinations  . natural  Graphs  show  neighbours  to a l l other combinations  T2, C) L3 o r T3, D) L4 o r T4.  replicates.  of  and  total  Lolium the  values  (stippled  involving  A l l values  yields for  perenne for bar  and the  4  graphs)  A) L1 o r T1, B) L2 are  means  of  3  TOTAL YIELD (g)  CA INDEX  TOTAL YIELD (g)  CA INDEX o  oo cn  _i i _  -I  I  I  I  I  L  J  1  I  L  O .— ( 1•  o  o  o  o  o  •XT •  •  1  1 —1  oo o  TOTAL YIELD (9) ro _j  o 1  00 cm » 1  TOTAL YIELD (g)  CA INDEX  J  NO NO  ro O  L_  00 cn  j  |_  J  I  I  L J  CA INDEX l_  0 0 O *-• 1-0L_. I  —1  O -tr  O  1  1  182  involving effects  Lx  Tx  (Fig.  compare  yields  them  are a l s o  apparent  although between  combinations  involving  involving  L3  four  o r T3  natural  combining from  Significant  illustrated  with combining  relationship  the  19).  were o b t a i n e d f o r L 1 - T 1 , L3-T3 and  Total  are  or  ability  there total L1  o r T1  (Fig.  neighbouring  ability  indexes  indexes. to  and  (Fig.  19c).  be  19a)  i n terms  to  strong trends  some  and  Examining genet  No  in order  combining  negative ability  in  in  combinations  t h e d a t a as a whole,  pairs  but d i d n o t  the o t h e r combinations  L4-T4.  i n F i g u r e 19  appears yield  (P<0.05) d i a g o n a l  had  the  significantly  of t o t a l  yield  highest s t a n d out  (Fig.  20).  DISCUSSION Three  out  were sampled higher came  of  significant  community capacity  a  than  o f L o l i u m and neighbours  those  i n which  neighbourhoods.  results of  pairs  immediate  ability  different  indication  genet  t o g e t h e r as  combining from  four  are s t r e s s e d  potentially  structure.  Combining  ability  to avoid competitive exclusion  coexistence  f o r one  genet  t e r m makes no a s s u m p t i o n defined  in this  t h e two  components  combinations the  inferior  or  lower  s t u d y by in  i n which  combination  as  the s u p e r i o r  component have a l o w e r  combining  may  ability,  components  three  be  or as  of  had  sets  of  experimental  thought  factor  in  of as  the  the p r o b a b i l i t y  relative  the d i f f e r e n t i a l presence  t h e two  organizing  t o mechanism.  the  field  they o f f e r  important  which  i n the  The  since  Trifolium  to another.  Combining  The  ability  i n the performance each  component  other.  vastly  probability  than combinations  of  of  is of  Those  out-yielded coexistence,  i n which  the  two  183  FIGURE  20.  Relationship  between  ability  index  for different  genet  perenne  and  neighbouring experimental replicates.  Trifolium genet field  pairs  total  type  yield  combinations  repens.  The  values  collected  from  the  are c i r c l e d .  and  combining of  f o r the  four  sites  Lolium  naturally in  A l l v a l u e s a r e means of  the three  184  x L1T3  24 -j A x L1T2  18  16 -  x  LAT1  14 -  CD  12 -  ®  x L3T1 x  L2T2  L4T3 x L2T3  Q _J  x L3T4  UJ  >-  ®  x L1T4  10 H  _J <  L4T4  x L2T1  x L4T2  r-  o  © L1T1 ®  8 H  L3T3  x L3T2  x L2T4  6 J  2 -J  — I 0.1 -  r~ 0.2  —r  -  0.3 COMBINING  —r— 0.4 ABILITY  —I— 0.5 INDEX  0.6  185  components were more s i m i l a r combining  ability  t o some d e g r e e escape  from  implies  in their  a high p r o b a b i l i t y  of n i c h e s e p a r a t i o n competition  w i t h more e c o l o g i c a l  separation)  have a h i g h e r t o t a l  less  ecological  (e.g.  Seaton  Snaydon  - i n Snaydon  The as  ability  the  ability  is  not  neighbouring  genets  performance  of  neighbouring Turkington  that  performance  ( w h i c h was  same  was  in  always always  attributed genet  combining  ability  genet.  was  parallels  least  ability  pairs  non-  that  highest  of  natural  that  the  with i t s natural the  to was  result  findings  genets),  but  lowest with i t s is  yielding  this  of  response to  yielding  i n genet of  to  combining  the  to.clover  The  consequence  these genet  had  highest  the lowest  t h e c l o v e r ) was  of  combining  to the f a c t  of the h i g h e s t  t h e g r a s s ) and  The  i s that,  indication  ability  each g r a s s genet  performance  neighbourhood.  higher  no  (1979c) w i t h r e s p e c t  of  Remison &  ecological  was  combining  (which  neighbouring clover  differential  (which  Harper  for  of  There  those  1980).  n e i g h b o u r i n g g r a s s s p e c i e s as o p p o s e d  the  natural  &  genet  & Adams 1969,  Higher combining  clover  niche  separation)  of t h e p r e s e n t r e s u l t s  interpretation  i s instead  grass  niche  of  i s that  (more  n e i g h b o u r i n g g e n e t s compared  20).  each  Allard  mechanism  justified.  (Fig.  result  ability  (less  Joy & L a i t i n e n  w i t h the h i g h e s t  yield  The  due  measure  i n c o m b i n a t i o n than  ability  feature  naturally  g e n e t s , an  combinations  natural  1978,  underlying  between  neighbouring  yield  & A n t o n o v i c s 1967,  most n o t e w o r t h y  concerns  total  combining  some  1977b). combining  Ecological  of c o e x i s t e n c e  affording  (Harper  combinations  with  performance.  that  component component  pairs is  the  from a  the  higher  but n o t a h i g h e r  total  186  yield. Higher  combining  ability  these  results  cannot  amount  of n i c h e o v e r l a p . a  relative  c o m p e t i t i v e power  species  ability  high combining Trifolium  which  t o be g e n o t y p e ability  ability  - Trifolium  genets  a  result  variously  of  several  in concert rather  a) P i f f e r e n t i a l  fitness  through  ability  appears  that  factors  that  in to in the  permit  that  permit  pair  of n e i g h b o u r i n g  a  neighbourhood for natural  factors  than  shifts  of n e i g h b o u r i n g L o l i u m -  from a d i f f e r e n t  may  a  pair  different  T h i s higher combining  instead  in  relative competitive  the  t h e same f a c t o r s  between  of any  requirements  specific;  between one  neighbours  reduces the d i f f e r e n t i a l  Moreover,  community. be  i n terms  for resource  common.  g e n e t s a r e not  combining Lolium  in  natural  interpretation  of s e l e c t i o n  have  appears  interpreted The  involve  two  form  be  between  most  high  in  the  neighbours  likely  operating  separately: local  competitive  ability  and  recombinat ion If  initially  the c l o v e r ,  i t may  community. genotypes the of  Within may  such c l o v e r in  grass i s generally  eliminate a local  resident genotypes  local  would depend on  clover  neighbourhood  would  The  manner  f o r reasons o u t l i n e d  (The g r a s s p o p u l a t i o n i n c) b e l o w ) .  stringent  from  however, a few  The most  ability most  may  not  clover against  descendents  imposed  by  and  selection particular  respond  in like  Grass genotypes  c o m p e t i t i o n from  the  challenging  e x a c t c o u r s e of  t h e t y p e of c o m p e t i t i o n  t h e most  genotypes  competitive  l e a v e the  frequency.  genotypes.  face  more a g g r e s s i v e t h a n  grass genotypes.  grass  therefore  several  possess challenging  particular  increase  the  the  would  locally  187  adapted  clover  genets that  h e n c e p e r f o r m more p o o r l y clover true  genets  competitive  fitness  'seek  out' neighbourhoods  competition present  is  the  to e x p l o i t of  selection  readily  propensity  capacity  o f T\_  from  impending  repens permits i t t o  genotypes  suggests  no member  in  which  i t is i n which  differentiation. instead  imposes  that  likely  In t h e  environment,  an o v e r b e a r i n g  of the genotype  are  threat ability  This  course  i f .niche r e q u i r e m e n t s a r e n o t  face  fitness  The  they  o f o t h e r s t h r o u g h any s u p e r i o r  i s especially  than  neighbourhoods  r e s o u r c e s ( a s i n (a) a b o v e ) .  partitionable.  competitive  greater  niche  i n the  fitness.  may be n e i g h b o u r h o o d s  through  fitness  contested  genetic  ability  and hence e s c a p e  These  however  a  alien  changes  e x t e n s i o n ) from  phenotype  3).  i n which  relative  have  of g r a s s  avoided  evidence  neighbourhoods on  may  competition,  (Chapter  of  of l o c a l  through migration  T h i s wandering  compatible  with  The c o n v e r s e would be  production  tracking  (through s t o l o n  stringent  than  and be i m p o r t a n t t o i n d i v i d u a l  genotypes  to migrate  mortality.  presence  The  facilitate  environment  Some c l o v e r  imposing  their  genotypes.  would  b) P i f f e r e n t i a l  others  in  from o t h e r n e i g h b o u r h o o d s .  for clover  recombinants  b e l o n g t o t h e same n e i g h b o u r h o o d and  of is  an  adversely  enhanced  once  to migrate through l a t e r a l  altered  a g a i n by a spread.  c) B e n e f i c e n c e between g r a s s and c l o v e r A beneficial nitrogen could  rich  effect  environment  be a s u f f i c i e n t l y  genotypes competitive  of  on t h e g r a s s may  grass  pressures  provided  by  i n t h e p r e s e n c e o f t h e legume.  strong  selective  which  do  on  be  the  not clover.  force exert  to  favour  highly  Grass  the This those  stringent  genets  which  188  ultimately  leave  t h e most d e s c e n d e n t s  therefore  be  competitive  e x c l u s i o n by  competitive  those  as  that  are  the  in l o c a l  competitive  suppress  e l i m i n a t e the  or  a neighbouring  especially  fitness  scarce.  The  mutualism and  may  Hill  strains  of  divergence.  fitness  if  lost  the  in  of  the  above s o r t  mutualistic  process. of  the  reciprocal  species exploiting  selection  i n the  operating  to  present  in  the  resources,  may  even  and  from also  Lolium  nitrogen  is  indirect  grass  the  vigour  a  in  adaptations environments.  both which  may  genotypes  i n the  clover  and  result  Trifolium and.' to  series  of  and  the  be  efforts  environmental as  (Chapter  evidence 5).  selection  for  S-selection  Grime reduced  ("stress-  brings  reproductive  e n d u r a n c e of c o n t i n u o u s l y  an two  may  vitalize  presumably  is  Local  populations  convergence  of  in  niche.  pre-empt  results  theory  of  on  higher  relationship  overlapping  S-selection  allow  c o n t r i b u t e to  aggressiveness  curb  vegetative  constraints  instead  phenomenon  his  strategy). in  not  involve niche  replacement  place  may  struggle  advocates  competitive  reduction  be  a complex  Rhizobium  may  a broadly  synchronously  respectively  tolerant"  may  local  or c o m m e n s a l i s t i c  Selection  competing  (1979)  of  in  particular  symbiotic  Niche divergence  the  equilibration  suggests  of  by  so  source  This  coadaptation mediated  not  1977).  A beneficence niche  further  involving interaction  particular  (c.f.  be  clover.  may  avoid  are  i f available soil  genotype-specific  neighbourhoods  turn,  to  in  n i t r o g e n made a v a i l a b l e by to  enough  c l o v e r but  to s e v e r e l y  important  neighborhoods  about vigour,  unproductive  189  The the of  relative  local  c o n t r i b u t i o n of  biotic  further  specialization  inquiry.  The  defining  selectional  Ecologists  have t e n d e d  separate  types,  e.g.  have  generally  interaction process  and  before  about  that  respect,  two  may  be  cooperating  in  simultaneous  intra  1981).  - specific  (Mather  Possible  cooperation increased  protection feeding  interference  ability  so  the  far  as  1961)  from w e a t h e r , efficiency, (Buss  cooperative  minimal c o n t r i b u t i o n to  risk  reduction  of  Evidence between  c a s e s where two for  the  in competitive  for concomitant  species  same  i t may  i s scarce. species  of  Yet  it  one  Empirical  and  animals  risk  is  in  for producing  mostly e.g. (Buss  include offspring,  of  increased  predation..,  interspecific  (1961) a r g u e s t h a t  i s e s s e n t i a l to be  requiring  invertebrates  favoured  in  fitness,  even  at  a  the  capacity.  p o s i t i v e and Thomson  negative  (1978,  1980)  entomophilous annual  pollinators,  not  cooperation,  in  Mather  function  certain  evolutionary  competing  and  reduced and  1981).  the  organism  concerned  in sessile  required  beneficence  1981).  have  functions  genders  and  another.  date  and  commensalism,  e x t r e m e s of  are  competition  beneficial  between  increased  to  interesting.  phenomena as  which  considerations,  Drosophila  Competition  Hamilton  in  interactions into  a c c o u n t s of  &  worthy  beneficence  predation,  organisms  theoretical  in  species  cooperative  developing  is a topic  of  conflicting  1960  (Axelrod  here  in  particularly  mutualism.  been v i e w e d as  attention  reasonable  is  c)  role  'pigeonhole'  l a r g e l y dismissed  special  possible  parasitism,  amensalism, competition,  and  reported  forces to  a ) , b)  but  where  they  interactions has  plants grow  described compete together,  1 90  pollination because  r a t e s f o r both  the  more  proportionately (1976) that be  cite  improved  spatially  more  visiting  numerous  the defense  be  reasonably  t h a t the  protection  from  For  the  O'Dowd  1976)  damage  from  stands  with  further  example  different  was  found  its  of  compared  i n a study the  that  less  removed  demands on  is also  necessary  from  of  .  a p o s s i b l e source  light, of  the  same  Pfeiffer gain  if  of  grasses  on  1970).  were  alone  w a t e r , and  of the It  were  of  the  extends  the  (1964) e m p h a s i z e d ,  o n l y one  A  removed,  presence  community  Harper  i n mixed  removal  & Harper  & to  stands.  vegetation  the  species  repens  pure  grasses  the  in Atsatt  decreased  the e f f e c t  the  a h a b i t a t i s not  limiting  in  (Putwain  than  of  &  Festuca  - cited  was  to  i n some way  members  some  competition  of T r i f o l i u m  d i c o t y l e d o n s and  n i c h e of Rumex a c e t o s a  component  (1972  neighbouring  Rumex a c e t o s a  increased  dicotyledonous  making  of  T h i s suggests  realized  it  size  population  other  i s given  t h a t i f both  removed.  Using  caterpillars  perenne  components  population  .  Radcliffe  g r u b s and  Lolium  offering  may  when a s s o c i a t e d w i t h  showed t h a t s u s c e p t i b i l i t y grass  other pests  neighbours  cattle  0'Dowd  suggesting  example, P h i l l i p s  as  study,  &  literature  g r a s s e s A g r o s t i s and  from  alone  attracts  Atsatt  whom some measure of  b u t t e r c u p Ranunculus bulbosus present  grow  bloom  h e r b i v o r e s and  noxious in  the  specific  assumed.  (1958) d e m o n s t r a t e d considerable  with  when t h e y  pollinators.  of a p l a n t from  i n t h e company of  than  concentrated  examples  measure of p r o t e c t i o n and can  are higher  "A  of many s p e c i e s  nutrient supplies;  micro-environmental  conditions  for associated species."  Because  of  the  high n i t r o g e n requirement  of many  grasses  191  combined w i t h  the  nitrogen-rich  legume, a n a t u r a l b e n e f i c i a l a  competitive  1978). of  such  as  T h i s may  than  result  start The the  years  the  growing very resultant  grasses  face  a  aggressively  soil  of  consider these  early  low  for  utilization supply  competitive  ability  increase  in  legume.  Those  this  regard  successively their  this  compatible  1950).  important  grasses  may  limit  year.  the growth of  and  cannot  i n the  more  favour that  relationship.  the  selection  those  both  "grassland  both  could  establish  a  more  As i t  is  possible  that  with  respect  to  regarding  i n s t e a d favour  potassium nitrogen  a decrease  accompanied ability  in  by an  in  the  which a r e l e s s a g g r e s s i v e i n survive  and  reproduce  good and p o o r c l o v e r y e a r s  c l o v e r neighbours  to  in  competitive  to  similar  I t i s reasonable  corresponding  genotypes of g r a s s  as  individuals  beneficence may  A  detailed  (1979a).  the  compete  so t h e c y c l e c o n t i n u e s .  should  able  In  growth because the  i n the grass  through  other  found  i s i n c r e a s e d ; hence,  differentiation  be  have been  by Robson & L o n e r a g e n  f o r potassium  should  with  'good'  supply  Harper  c o u l d suppress  to the grass,  together  During  f o r potassium  populations  niche  the  (hence a 'poor' c l o v e r y e a r ) .  &  competitive  by  i n the s p r i n g of the subsequent  potassium;  selection  fluctuating  selection  (see review  nitrogen  Turkington  'balancing'  for  c l o v e r a t t a i n s more v i g o r o u s  for  that  clover  nitrogen  phenomenon has been d e s c r i b e d cycle"  occur  grasses  " ( B l a s e r & Brady  competition  year,  provided  i n what has been d e s c r i b e d as t h e " c y c l e  leguminous a s s o c i a t e  following  may  Often,  white  potassium  good and p o o r c l o v e r  clover  interaction  one i n p a s t u r e s .  t o be more c o m p e t i t i v e nutrients  environment  and t h e r e b y  may  along leave  more with more  192  descendents. As these  this  grassland  amplitude present in  or  cycles  study  than  higher  surveys  continues  during  might  expected  frequency. can  be  i n the  in Figure  had  1981  percentage  be  Evidence  seen  a l l three pastures  surveys a  selection  1979  surveys.  have s t a r t e d  i n the  1977  pasture,  and  compressa  tapers  a l s o showed t h i s  1977  pasture  1958  pasture. Biotic  in  the  and  s t r u c t u r e and  occupying  a  and  unit  et  .  organisms interact present (Chapters  of  area,  regards  1981).  with  4  had  i n the  1981  barely  1958  pasture.  Poa  as  3 ) . " The  another  (Odum as  on  a  the  since  any  affect 1971).  the  local  findings  the A more of  o r g a n i s m as  the  MacMahon to  sessile  individual  neighbourhood. reported  t h a t e v o l u t i o n a r y changes  of  'montage'  suited  given  view  populations  MacMahon e t a_l 1978, is especially  the  traditional  of  to  in  demonstrated  - centered  thought  centers  to  i n the  same t r e n d  collection  in only a very  corroborate 5)  1939  level  community  (eg. view  perenne  seems  organism  the  which  plants  others  and  f o r an  the  This  as  results  genotype  usually  interaction  such  the  as  1979  t r e n d to c l o v e r i n the  e v o l u t i o n (Chapter  neighbourhoods  pivotal  cover  calls  i n the  i s most p r o n o u n c e d  opposite  the  repens  cover  the  in  cycle in  2).  cycle  in  at  a  three pastures  The  abundance of one  approach  evolving  the  somewhat  community  a . given  distribution refined  study  (Chapter  l a n a t u s d i s p l a y e d the  specialization  defines  al  Holcus  present  community view  off  such  diminish  In c o n t r a s t ,  in  i n the  to  percentage  surveys.  than  pasture  2  a higher  cover  for  community e v o l u t i o n ,  i n the  will The  earlier combining  193  ability  o f s p e c i e s may  differentiation, abilities. genetic that  also  The p r e s e n t  study  variation,  consequence a  of  different  of  indicates that,  micro-evolutionary determine  reciprocal  opposition.  which  niche  competitive  given  sufficient  be so p r e c i s e adaptation  in  even a t t h e s c a l e The  f o r more i n v e s t i g a t i o n s o f t h e interactions  only  of  f o r c e s may  g e n o t y p e s of t h e same s p e c i e s .  species  not  'balancing'  n e i g h b o u r h o o d s may be n e i g h b o u r s p e c i f i c  e x p o s e s t h e need role  a  but  the p r o p e r t i e s which  local of  be  interpretation occurrence  integrate beneficence  and and  1 94  CHAPTER 7  GENERAL DISCUSSION  195  PROSPECTUS  The this  results  thesis  call  coexistence  in  Chapter  the  1,  concepts  of  latter  minimal  the  stages  .in  This  between  operational  association individual  inter-specific this  community changes  to  was  set  the  niche  competitive  ability  is  and  the  parlance  revealed  of  time-series  trends  pastures  which  under  is  further  corroborated  presented  species  are  therefore  A  of  which a t t r i b u t e s  9). the  forces  Armed w i t h central  responsible for this  The  represent  the  trend  selective  3  represent  time-series (Fig.  was  an  dynamics  of  patterns  'of  level  of  stability  of  discovered  'portrait'  of  within-community accruing  this  'settling  this  and  pasture temporal  from  information,  i s s u e of  8).  reflecting  fine-scale increasing  the  community  by  i n t e r p r e t e d as  interaction.  at  support  of  in Chapter  and  percentage  study  progression  of  with  competition  common  coexistence  (Fig.  f o r t h e meaning  t h e s i s to proceed  o r d i n a t i o n of  individuals  for addressing  mechanisms a r e  between  a basis for constructing a  the  interactions  distinction  sampling  evolution  of  overlooked  a s s o c i a t i o n in older pastures  i s u s e d as  phenomenon  In  a  experience.  in  plants.  2,  . contacts and  the  presented  between  three  measure of  interspecific  competition  the  4)  that  of c o n t a c t  Contacts  in  (Fig.  development.  work  d i s c u s s i o n of  This enables  In C h a p t e r  suggestion  the  a comprehensive p e r s p e c t i v e  ambiguity  different  of  largely  is offered.  surveys  surveys  for a general  systems  and  coexistence. cover  i n t e r p r e t a t i o n s of  fundamental  established the  and  biotic the  stage  thesis:  What  down' of  community  196  d y n a m i c s and It other  is  tacitly  through  dominants 1979).  important  their  (e.g.  in  1969,  Watkin  Harper  as c o e x i s t e n c e  factor.  The  the  present  biotic  competition  will  factor  for  relationship  that  studying  on  time,  the  an  how  pair  of  a  as  single  hypothesis especially  of  community  opportunity  the  in  Grime  phenomenon  stages  precisely  between a p a r t i c u l a r  a  1978,  interactions,  same  and  potential  e x p l a i n e d by  With d i f f e r e n t the  grazing)  & Clements  fully  of n e i g h b o u r  at'  (i.e.  suppressing  work t h e r e f o r e f o c u s e s  available  available  be  systems?  permitting diversity  in ecology  rarely  i s important.  development  factors  effects  It i s a l s o axiomatic  broad  i n these  assumed t h a t p r e d a t i o n  disturbances are  pastures  that  permitting coexistence  was  competitive  s p e c i e s changes  during  community e v o l u t i o n . Natural competition selection designs  selection and  for  other  combining  'combining  corroborative ability  'ecological'  abilities). coexistence  may  combining  'competitive'  The  neighbour  to  ability'.  theory  arguments  ability'  ability  are  presented  between  (Chapter  1).  the  be  defined  (Chapters  4,  (balanced  these  here  important follow concepts  f i n d i n g s to  in relation  to  f o r c e of n a t u r a l from of  5  indicates  contrasting  recognizing ' n i c h e ' and  of as  experimental and that  types:  (niche d i f f e r e n t i a t i o n ) , ability  pursued  i s an  two  in contexts  three  Evidence.  of  r a m i f i c a t i o n s of  competition  distinction  be  The  interactions  data.  combining The  coexistence  i n t e r a c t i o n s may  for studying competitive  6) . y i e l d e d  which  leading  and  1) 2)  competitive contemporary systems  in  selection. a  clear  'competitive  1 97  COMPONENTS OF A GENERALIZED COEXISTENCE THEORY  A theory out  below  f o r the coexistence  which  interpretation previously the  integrates  of  the  fundamental  A qualitative  described  by t h r e e u n d e r l y i n g  Gaussian The  by  theoretical  the contents to  first  a  2)  broader than  incorporates  components;  relative  1)  competitive  in Figure  24  and i s  concepts.  in  the  component,  &  Levins  proceeding  species.  (although  principle  implicitly  component  -  sufficiently  i.e.  Given  never p a r t of invokes relative  different.  an  if  (a c o r o l l a r y  i s p o s s i b l e only  if  i s in  In r e g a r d t o  fundamental  niche  ( i n t h e sense of ' l i m i t i n g Gause's  principle  toward c o m p e t i t i v e  exclusion  t h i s c o n d i t i o n f o r the f i r s t any  formal  assumption  statement) f o r the  abilities  difference present  in  niche  be  competitive any  two  i s complete.  of the c o m p e t i t i v e  fundamental  the  second  will  between  s p e c i e s u n t i l e x c l u s i o n o f one s p e c i e s  coexistence  i s denoted  which  system.  competitive This  21  1967)),  i s assumed t o be i n v a r i a b l y  interacting  Figure  h y p o t h e t i c a l two-species  component,  principle)  and  i s portrayed  of t h e d a s h e d box  (MacArthur  of  Gaussian  primary  a  selection  framework o f G a u s e ' s p r i n c i p l e  pronounces a r e l e n t l e s s  abilities  two  are s u f f i c i e n t l y similar  similarity'  one  model  theoretical  requirements  of  natural  i s set  coexistence  reference the  of  requirements,  ability.  A)  of  with  I t s e v o l u t i o n a r y premise  variability  niche  competitors  t r a d i t i o n a l theory  operation  recognized.  d y n a m i c s and  of p l a n t  exclusion  requirements  198  FIGURE 21. species  The  coexistence  important  force  recognizes niche Three  two  of  concepts  evolutionary  selection  for  differentiation) combining  ability  and  and  distinct  (2)  'Gaussian'  ecological and  (coevolution)  and  general  where  The  discussed  permit  combining ii) (C).  general (1)  competitive  coexistence  which  theory  competition  components:  Relative  mechanisms  (B),  a  selection.  represented  for  of  contexts  natural  are  theory  structure  in  basic  requirements,  traditional two  schematic  selection  an  theory  Fundamental abilities.  i n the  (dashed  text: box);  coexistence ability for  is  of  (A) and  -  i)  (niche  competitive  (B) SELECTION FOR ECOLOGICAL COMBINING ABILITY reduces fundamental niche overlap  -a  r  -i  (D  FUNDAMENTAL NICHE REQUIREMENTS sufficiently similar  (2)  (i)  (A)  RELATIVE COMPETITIVE ABILITIES sufficiently d i f ferent  FUNDAMENTAL NICHE REQUIREMENTS sufficiently d i fferent  (2) (compet i t i v e i n t e r a c t ion absent in exclus ive n iches)  J  (C) SELECTION FOR COMPETITIVE COMBINING ABILITY reduces difference in relative corq)etitive abilities  200  are  different  permitted), do  not  niche  the  crucial  always  space,  Gaussian  either  to  exclusion That  (1982):  broadly  one  eliminated  philosophy  fittest'.  species  by  of  the  to  a  for  is  surprising  really  i s that  Sheppard  this  nature  f o r ) the driving  (Gause  species  in  pervasive  in current  -  too  of  they  similar  from  Newman  t o c o e x i s t more  must  show  a l l but  difficult  to  one  will  imagine  central  'survival  the  inherent  T h i s was  1934,  physical selection"  of  on the  Wallace  capacity  competitive 1948,  out,  of  within  use  has  the  exclusion  1954).  largely  the  combat, and  could  f u r t h e r r e i n f o r c e d by  Park  pointed  be how  focus  themes of D a r w i n and  of  niche  Gause's p r i n c i p l e  e x i s t e n c e ' and  immense v a r i a t i o n at.  quote  same theme p e r s i s t s  (1975) has  which suggests  "all-embracing  the  competitive  in order  s t u d i e s demonstrating  laboratory populations  potential  that  outward  in size.  in  'struggle'  two  to  the  recent  I t i s not  awareness  increase  experimental  As  species  According  early evolutionists'  'struggle  from t h e i r  populations  today.  are  Such were s t r o n g  conceived  two  is  some e x c l u s i v e  between  is s t i l l  statements a s s o c i a t e d with  as  the  niche,  s p e c i e s must d i f f e r  have been a p r o d u c t  was  occurs  overlapping  in competition".  ideas  that  overlap  i s impending.  If  such profound  amount of  temporally.  " I t i s generally accepted  separation.  early  or  thinking i s evident  transiently  such  'limited'  consequence being  spatially  the Gaussian  ecological  a  where c o m p e t i t i o n  a  of  (i.e.  have t o compete b e c a u s e e a c h has  view,  respect  than  'enough'  unaltered  of  the  word  obscured  the  importance  species  What  that  of  (and  Darwin  201  B)  Coexistence  (niche  by  s e l e c t ion  genetic  recognizing variation  ecologists enables  turned one  that  their  species exclusion)  1961,  & Cooke  Mather  natural  combining theory,  this  species  pressures  of  interaction  win  i n the  been  results  through niche this  evolutionary  in F i g .  21).  and  interspecific  exclusion  will  combining  ability  principle  (c.f.  is  an  Grubb  If  is  each  role  by of  plant  ecological Under  the a  this  selection  competitive  p.107) and  density, natural  is  coexistence sufficiently relaxed  remains,  selection  or  framework  1)  become  overlap then,  explaining  'Gaussian'  competition  niche  1977,  to  assumes t h a t :  extension  and  a  requirements  In e s s e n c e  differentiation  exclusion  niche  where  ensue.  inter-specific  niche  It s t i l l  if  2)  In  for  approach  within  achieved  and  the  Ho  differentiation  1977b).  in avoidance  &  differentiation.  ( d a s h e d box  eliminated)  Lerner  selection  evolution,  what  interaction  21-B).  1967,  because  of  investigating niche  have  requirements,  1955,  (Fig.  termed  entrenched  high  1964), t o  1964,  coexist  hence  & Gotoh  coexistence  is s t i l l  different  a  e v o l u t i o n of  coexistence  only  in Sakai  Gale  (Harper  that  fundamental niche  competitive  competition,  Notice  is  ability  usually  to  has  ability  will  investigations  (e.g.  permitting  interactions  population  a t t e n t i o n from  1962,  selection  hence  a  p e r t a i n i n g to  (competitive  and  combining  differentiation)  Upon  of  for ecological  for  corollary presupposes  selection  hence c o e x i s t e n c e  may  - else,  (or  competitive ecological of  Gause's  that  under  result  in  competitive  forthcoming. species  i s a f f e c t e d more by  the  presence  of  the  202  other  than  by  competition species  relative  In  be more a g g r e s s i v e  may be s t r o n g fitness  different  competitor  presence  intraspecific  selection  competitor  when  difference large,  would  here  faced with  i n the s p e c i e s '  (displacement)  may  be no r e a s o n  competitor.  result  will  comes p r i m a r i l y  from  From  standpoint  preventing  most  important  exclusive  requisite  niche  permitted  virtually niche  f e a t u r e of t h i s  as  competition  the The  a  weak I f the  abilities  s t r o n g and n i c h e competitor,  individuals  is  shift  but  there  i n the stronger  for  the  of. i t s own  competitive  each  is  mechanism  long as c o m p e t i t i v e  differentiation in  on  stronger species.  e x c l u s i o n , the  i s t h a t t h e weaker c o m p e t i t o r  e l i m i n a t e d and t h i s  resulting  by  competitor.  pressure  of  secure  an  space.  The c e n t r a l is  of  weaker  1977b).  exerted  displacement  competitor the  be  be  in plants  presence  competitive  competitive  divergence.  competitor  stronger  will  niche  the  in  occupying  the  (Harper  i n t h e weaker  t o expect  i n niche  stronger  relative  differences  species)  by  One  but d i r e c t i o n a l  unequal,  more  a much  selection  Significant  are  then  1977b).  of c o m p e t i t i o n  individuals  pressure  directional  result  type  and t h e  and  ( i n each  affected  individuals  greatest  the other,  abilities  generally  individuals,  (Harper  species  'exploitation'  interspecific of  both  s p a c e may  competitive  is  than  individuals  of n i c h e  t h e more common if  in  amongst  regions  however,  of i n t r a s p e c i f i c  i s of the ' i n t e r f e r e n c e ' type  may  selection  the presence  when  having  avoided,  is  interaction  i s accomplished competitors an  more  that  by  coexistence i s reduced  selection  encounter  one  exclusive  niche  space.  efficient  exploitation  or for  another If of the  203  environment yield  in  Evidence pairs  results  species  after  in  the  be  diallel  limited  by  16).  the  become n e a r l y  list  1978,  den  (Fig.  Holcus  potential  pre-empting  axiomatic  numerous t o van  of  (see  in recent  coexistence divergence  in  niches.  1974,  1978).  (Wiens  Coexistence  by  1977,  Connell  the  guilds  species  and  and  generate  novel  to support  multi-species  selection.  11)  ecological  Bergh & Braakhekke  total  in  the  Trifolium  i n t h i s manner w i l l  to  f o r example Cody  empirical basis  higher  lanatus  new  be  genetic  This  theme  has  far  too  studies  Grubb  1977,  Diamond  Most e v i d e n c e  however  c i r c u m s t a n t i a l ; some have r e c e n t l y a r g u e d  is- l i t t l e  a  i n t e r p r e t e d f o r some  Selection operating  species'  capable  have  study  series analysis for  (Fig.  variants  mixture  will  s e l e c t i o n than b e f o r e  f o r t h i s mechanism may  repens  C)  two  combination  replacement  is  and  that  in fact  there  commonly h e l d view is  a  result  of  that niche  1980).  s e l e c t i o n for  competitive  combining  ability  (coevolution) Genetic pertaining  variation  not  only  will  to p a r t i c u l a r  requirements,  but  ability  common n i c h e  force  f o r any  of  genetic suggests is  very  will states  also  competition variation. that large  be of  the  will The  a l l the  in  exhaustive  a  on  relative The  b o t h of  population  Indeed, a vast  on  selectional  components  plant  plant  which determine  of  genetics  characters,  number of  1 (Chapter  niche  competitive  same  these  genes c o n t r o l l i n g  in Table  a  fundamental  literature  population  characters  to  in  of  requirements.  operate  1980).  present  types  pertaining  number of  (Solbrig  present  be  the  1) w h i c h  alleles various govern  204  competitive in  ability.  Possibilities  considering that  a r e even  one g e n o t y p e may  f o r one n u t r i e n t w h i l e  have  uptake e f f i c i e n c y  greater  will  have t h e s e l e c t i v e  the  prevailing  natural plant that  selection population  competitive  'character' Under a  t o improve seem  operate  populations  for coexistence,  a c t s as  The c r u c i a l reciprocally so  any  alternates  between  as  characters  and  constantly members need  respond  not  exclusion; acquire,  itself  as  a  agent  on  i t s own  i n . two  pressure competing  ultimately loses i t s therefore  (and amongst) members o f t h e two  populations  which  confer  recombined, once  competitive  again.  Local  t o one a n o t h e r . competition  i t may  instead maintain  in  order  once  to  avoid  the p o t e n t i a l  gene  flow  competitive  selection,  that a l t e r s  A species population  under  the p r e v a i l i n g  are  neighbourhoods  i n a fine-tuning process  mutation,  ability  selected for, selected against,  v a r i a n t s with  natural  a  any g e n o t y p e t h a t i s  genotype  p r o p a g a t e new g e n e t i c  of  in  control.  sequentially  avoid  through  ability  such s e l e c t i o n  'Superiority' in  for  engaged  and  genetic  i s that  of  (1961) h a s p r o p o s e d  selective  point  nature  competition  combined,  selected  a  'superior'  advantage.  generated,  Sakai  this  competitive  various  competitive  partial  competitors. will  relative  i n p l a n t s c a n be t r e a t e d  mechanism  may  The o p p o r t u n i t i e s f o r  under a t l e a s t  superior competitor  genotype  depend on t h e  pressures.  innumerable.  ability  a second  f o r a second n u t r i e n t ; which  advantage w i l l  competitive  multiplied  f o r example have a g r e a t e r  uptake e f f i c i e n c y a  further  or  again  the  the  way  therefore  competitive  to generate  (or  recombination)  and  increased competitive  pressures.  are  In each  strongest  such  power event  selection  205  pressure  will  be  exerted  weaker c o m p e t i t o r . selection  also  concurrently competitor  that  comparable  each  under  selection  time  abilities  to  reduce  ability' yield  be  (c.f.  in  14).and  combinations  species  of  is terms  perpetual might  not  these  two 6  important  under 'fight'  this  is  more  6).  by  reciprocal  both  species,  competitive e x c l u s i o n of  Lolium  after  with  alien  for the  analysis  neighbouring  p e r e n n e compared  from  Total  selection  the d i a l l e l  of n a t u r a l  this  combining  ability).  consistent  in  to  genet  neighbourhoods  19).  t o be m i s l e a d 'weaker'  mechanism  i m p l y i n g an  imagine  increase  species  ' s t r o n g e r ' and  stronger  for competitive  ability  and  (Fig. not  within  of may  ability  in r e l a t i v e  combinations  repens  the  (Chapter  combining  This  i n the c o m b i n i n g  i n Chapter  selection'  one  5  of T r i f o l i u m  It  (=niche)  ability.  (Table  the  ecological  least  Following convention,  'selection  combining  for  by  termed,  in  to c o m p e t i t i v e  21-C).  need  results  discussed  (Fig.  lead  at  selection  i s achieved  difference  combination  competitive  genets  the  otherwise  t h e weaker c o m p e t i t o r may  mechanism  operating sequentially  w h i c h might  mechanism  this  weaker  I f some form  competitive  weaker c o m p e t i t o r  the  natural  the  ability  variants  overall  on  if  in  competitors,  genetic  an  t o t h a t of t h e  variants  competitive  between  those  have  Coexistence  genetic  stronger competitor.  occurs  favour  competitor  exclusion i s avoided  overall  t o t h a t of t h e  beneficence  natural  those  t h a t have an  comparable  t h e more p o w e r f u l  Competitive  favours  competitor  by  is  i n t h e above d i s c u s s i o n  competitor. not  increasingly  i n the c o n t e x t  an  'arms r a c e '  costly  of a g g r e s s i v e  'Reciprocal  enterprise  in a as  interference in  206  animals. more  In c o n t r a s t ,  i t i s p o s s i b l e t h a t any  subtle characterstics in plants  may a f f o r d  an  different  times,  competitive  individual  a  pressures,  of  several  1, C h a p t e r  edge.  Moreover,  a g e s and under d i f f e r e n t  different  charactersitics  important  in  or  suites  may  'staying  S e l e c t i o n i s t h e r e f o r e not u n i d i r e c t i o n a l  multidirectional  and one need n o t t h i n k  out'  or  of  genes  Development the  Any  o f more b a l a n c e d  replacement  perenne  characteristics  series  from t h e d i f f e r e n t  of a s e c o n d  changes  in  acquired  species w i l l  the  second  competitive  abilities  of Holcus  aged p a s t u r e s  measure o f s u p e r i o r i t y  fitness  for  of  but  'running ability.  is  lanatus  (Fig.  at  competitive  of a p o p u l a t i o n  competitive  analysis  providing  1)  types of  characteristics power'.  be  (e.g. Table  competitive  at d i f f e r e n t  one  seen  in  and L o l i u m  16c, C h a p t e r 5 ) .  by one s p e c i e s a f f e c t i n g t h e  i n t u r n be t r a c k e d  species.  The  result  by  genetic  is  ongoing  coevolut ion. Competitive the for  pressure  of c o u r s e  competitive  combining  interpreted  specific  competitive  effect  means  ability  In f a c t ,  largely  by  this  combining  that  average  they  individuals high  difference  i s comparatively  d e n s i t y and  individuals  resource  will  process. ability coexist  in small  operate  members  both  of  i n t r a - and  coexistence  Selection  operating  species  in  ability  to experience  a  which  between  r e g a r d l e s s of s p e c i e s . such  be  continue to  species'  competitive  can  for inter-  i n two  because they  a 'single  limitation,  would be e x p e c t e d  from  s p e c i e s and s e l e c t i o n  intra-specific  behave c o m p e t i t i v e l y a s e s s e n t i a l l y the  come  same s p e c i e s a s w e l l a s from d i f f e r e n t  inter-specifically.  in  will  Under  collection  a process  of  analagous  207  to  'self-thinning'.  the  community  specific  and  From t h e  there  i s no  broadly  consequence  to  another  the  As  abilities  pointed  ability  out  i m p l i e s an  Measuring  the  measuring  the  ability'  (Harper  mixture,in  earlier,  type  yield  1964).  the  occur  components  and  equally  the  to  combining ecological  assessed  yield  in  total  ability, combining  In  ability),  with  combining mixture.  i s analagous from  a  'genetic of  of  a  cross  species  yield  contribution  in yield but  for  in  remain can  be  special  the  two  to c o n t r i b u t e competitive  a l s o occurs  only  two  variability of  potential  selection  is a  the  to  combining  ability  total  account  their  increase  any  relative  a  combining a b i l i t y  of  yield.  an  of  f o r combining  relative  is a reflection  of  of a g e n e t i c - c r o s s , t h e  which takes  the  their  each to the  Competitive  for  identity.  i n the product  c o n t r i b u t i o n of  ability  their  resulting  product  ability  for ecological  total  in  inter-  competition  of a s p e c i e s m i x t u r e  the  being  separately.  may  in  between  only matter  in  species  selection  However,  c o n t r a s t with  of c o m b i n i n g  which  with  homozygotes f o r a s s e s s i n g  and  measured  faced  of a h e t e r o z y g o t e  components w h i c h a r e distinct  The  difference  increase  yield  two  the  'individual'  combining  niches.  r e g a r d l e s s of  total  between  competitive  individual is  the  to d i f f e r e n t i a t e  overlapping  individual  competitive  reason  intra-specific  species with  p e r s p e c t i v e of  the  (as  with  inferior  component. Based competitive  on  21.  above c o n s i d e r a t i o n s , i n t e r p r e t a t i o n s o f  relations  investigations Table  the  between  reported  in  species this  from  thesis  the are  the  experimental summarized  in  208  TABLE 21. between  A  summary of  species  reported  interpretations from t h e  of c o m p e t i t i v e  experimental  relations  investigations.  EXPERIMENTAL DESIGN USED FOR INVESTIGATING COMPETITIVE RELATION  CHAPTER IN THIS THESIS  DACTYLIS GLOME RATA / HOLCUS LANATUS  Diallel  if  DACTYLIS GLOMERATA / LOLIUM PERENNE  Diallel  if  none (Competitive Exclusion)  DACTYLIS GLOMERATA / POA C6MPRESSA  Diallel  if  Compet i t i ve  DACTYLIS GLOMERATA / TRIFOLIUM REPENS  Diallel  if  none (Competitive Exclusion)  Dial l e i  if  Compet i t i v e  5  Compet i t i ve  SPECIES PAIR  HOLCUS LANATUS / LOLIUM PERENNE HOLCUS LANATUS POA COMPRESSA  /  HOLCUS LANATUS / TRIFOLIUM REPENS LOLIUM PERENNE / POA COMPRESSA  LOLIUM PERENNE / TRIFOLIUM REPENS  Replacement  Ecological  Diallel  if  Ecological  & Competitive  Diallel  k  Ecological  6 Competitive  5  Ecological  6 Competitive  Replacement  Series  Diallel  if  Compet i 11ve  Dial l e i  k  Compet i 11 ve  5  Compet i 11ve  6  Competitive  if  Compet i t i ve  Replacement Reciprocal  POA COMPRESSA / TRIFOLIUM REPENS  Series  INTERPRETATION OF THE TYPE OF COMBINING ABILITY RESULTING FROM SELECTION  Series  Transplanting  Dial l e i  210  OTHER MECHANISMS FOR  The  mechanism  ability same  of  selection.  and  between  necessary  to point  competitive  that  out  combining  environmental  the  again  1948,  Gulmon  may  result  predation  Huston  (etc.)  competitive  species  has 1955,  network of example  system  relative  may  be  based  on  and  a  i t is for  is  mediated  by  competitive  such  1977,  as  seasonally)  of  other  vary  Fagerstrom may  then  advantage  species may  with  (Hutchinson coexistence' disease which  or  would  (Caswell  a l s o be  1978,  possible i f that  each  p h a s e s of  the  life  cycle  & • Agren  1979).  be  age  the  so  p o s s i b l e by  abilities  a superior competitor  populations  another  competitors  in d i f f e r e n t  competitive  (e.g.  disturbance,  Coexistence  coexistence  environment  'Non-equilibrium  superior  exclude  i n the  abilities  the  abilities  advantage  Grubb  multispecies  natural  selection  variation  s p e c i e s , then  factors  to suppress  relative  (Watt  the  from  ability  one  1982).  P i c k e t t 1980).  an  by  proposed  differ  competitive  has  competitively  1979,  been  continually altering  Fowler  when  serve  otherwise  by  relative  1979,  also  for  ability.  i t , where f i r s t  first  have  they  combining  i s maintained  coexistence  c o n d i t i o n s , temporal  prevailing  competitive  between c o m p e t i t o r s  balance  competitive  coexistence  inhabiting  how  for  mechanisms  relative  allow  this  competitors  1) N o n - e v o l u t i o n a r y As  'balance'  O t h e r mechanisms of  'balance'  may  selection  i m p l i e s a tenuous  resources  COMPETITIVE BALANCE  .  a  Species  to s p e c i e s B while  In  a  circular A  for  species  21 1  B  i n turn excludes  species  A  (Pianka  some e x t e n t  the g e n e t i c a l l y  selection  ability in  Models  for  coexistence  factor  Pimentel  .species in  species  T h i s would  the but  will  natural  of o n g o i n g  the  Leon  argues  mechanism  that  1970).  stable  1965, A y a l a Levin  in  and  of  &  t h e r a r e r o f two  in intraspecific  interspecific Hence,  selection  for  likewise  the  competition.  genetic  feedback  b a s i s f o r such r e v e r s a l s i n  when a t l e a s t  performance  at  one  of  the  loci  ( i n e a c h s p e c i e s ) , one o f  interspecific fitness,  of  encounters.  competition  A genetic  competitors  1974,  . stronger  l e a d by a p r o c e s s  intraspecific  of  et. a_l .  in intraspecific  f o r improvement  stability.  (Ayala  of  mechanisms  1969, 1971,  experience  h o m o z y g o t e s has maximum  homozygote  of r e l a t i v e  role  balancing  (e.g. Pimentel  interspecific  competitive  minimum  Such  ability.  possible  (1968)  s p e c i e s dominance may e x i s t controlling  to  a r e not c o n c e r n e d  the  be i n v o l v e d more o f t e n  theoretically  ecological  they  study.  which s e t s apart  dependent as  and t h e commoner  improvement commoner  present  or  combining  frequency  species w i l l  with  selection  of c o m p e t i t o r s  rarer  competition  a d j u s t i n g i t in a process  considered  1977).  encounters  i n the  populations  1970, 1971, 1972, L e v i n  competing  to  in  dependent  have a l s o been  the  system  for competitive  2) F r e q u e n c y  in  dynamic and v a r i a b l e p o t e n t i a l  It i s this  selection  Udovic  wins  T h e s e mechanisms may be i m p o r t a n t  continually  coevolution.  1966,  which  however a r e n o n e v o l u t i o n a r y ;  competitive  of  1979).  i n the pasture  explanations with  species C  competitive  and v i c e v e r s a  ability  f o r the other  212  There a r e important  assumptions  d e p e n d e n t model t h a t a r e n o t f o u n d for  competitive  populations wide  combining  are purportedly  oscillations  ability.  maintained  animals;  i n nature  such o s c i l l a t i o n s  sessile  higher  come  plants  from  overlap,  i t implies that  ability  may  opposition  i n respect  make i t a good  effects  are in fact  plants  intra  is  -specific  density, improved apparent  inter  in  -specific  reason  that  reduced  intra  -specific  has  rapid  uptake  a  supply ability of  f o r example, over  species.  models  a plant  population  should  individuals  more  for  will with  directly  evidence f o r  populations  of  been o b s e r v e d i n  degree  of  niche  competitive  for interspecific resources.  individual from t h o s e and  that  a  good  attributes that  their  this  i s common  s p e c i e s which  i s a good  high may  inter  -specific  indeed  result.in  a b i l i t y ,• but. necessarily  competitive  this  an  To s u g g e s t  competitive  rate  of  competitor  ability.  If  there  is  no  also  result in  an  individual  n u t r i e n t s which a r e i n l i m i t e d  confer  a  a slower  L a w l o r a n d Maynard S m i t h  are  competition  make  but e x p e r i e n c e s  it  dampening  Moreover, as t h i s  the  different  In  the  a  have n e v e r  to selection  conflicting.  selection  two  laboratory  intraspecific  competitor  former,  to  1978).  selection  the  t o t h e same c o n t e s t e d  are  unfounded.  frequency-  for intraspecific  which  competitor  this  The o n l y  regarding  selection  implies that attributes  that  in  in  ability  interspecific  due  (Antonovics  mechanism makes no s t i p u l a t i o n  It  In  of s p e c i e s dominance. have  competitive  into  i n t h e mechanism of  such o s c i l l a t i o n s  be  built  superior uptake  (1976)  applicable  i s m e d i a t e d by b e h a v i o u r a l  to  rate, argue cases  competitive regardless that  such  in  which  i n t e r f e r e n c e (as i n  many  213  animals) as  rather  than  i s more common  here  involves  refers  merely  in plants.  'a-selection'  explicitly  plants  ...".  It  possible  exception  et  al  all  i n nature  .  In  frequency  escape  some  have n i c h e theory  from g a i n i n g a c c e s s  dependent  i n Chapter dependent  .e.g.  from  above.  the  With niche  pressure this  is is  competitive with  dominance  This  interaction  no and  forces there  that  which is  no  on  in plants i s two  species  (i.e.  Figure  12  they and  f o r example,  replacement  series  and T r i f o l i u m  repens  how  competition  intraspecific  always  this  discussed competitive  competitive suffers  w i t h an i n d i v i d u a l of the other  focussing  to understand  dependent  species  1971).  i s evident, in  Webb  be i m p o r t a n t a t  (See  n o t t h e same a s i n t e r s p e c i f i c each  (e.g.  each other  lanatus  differentiation,  because  with the  phenomena  with  i s important  frequency  an i n d i v i d u a l  are  5).  Holcus  5). ' I t  i n nature  perspective  1977b).  behaviour  between  feedback  (Levin  competitive  d i f f e r e n c e s ) (Harper  16c, C h a p t e r  differs  above  with  genetic  a mechanism s h o u l d by some  to the  difficult  by a l l e l o p a t h y  a consequence of the f a c t  presented  analysis,  be i m p o r t a n t  measure o f c o m p e t i t i o n  frequency  there  such  stark c o n t r a s t to the  considered  than  That  i s viewed u n l i k e l y  usually  (Fig.  should  ability  o f i n t e r f e r e n c e phenomena,  especially  of a u t o - t o x i c i t y  1967).  animals,  but  therefore  f o r competitors  of r e s o u r c e s  (1974) w h i c h "...  t o i m a g i n e an i n s t a n c e where P i m e n t e l ' s  mechanism  from  d i s c u s s e d by G i l l  a competitor  is  exploitation  Selection forcompetitive  to the a c q u i s i t i o n  mechanisms w h i c h p r e v e n t resources  relative  pressure  more  from  o f t h e same s p e c i e s  species.  should  produce  reason  to  In t h i s  situation  oscillations  invoke  in  an e x p l a n a t i o n  214  involving  any  specific  competitive  dependent than  here  overlap  frequency  because  intraspecific  competition  between  in  are  between  different  interspecific  In  that  genetic  favours operates  interspecific  competition,  according  combining to  the  inter-specific,  genetic  of  feedback  more  model  nature  behavioural  intraspecific  intense  Competition  i s of a d i f f e r e n t  feedback  than  interference  individuals  against  important characters  and  ability,  mechanism,  vice  versa.  competitive  prevailing  competitive  and no a s s u m p t i o n  is  in intra-specific  competition  even  any  from,  different  in  the  than  important In. s e l e c t i o n is  pressures, that  in for  improved i n t r a - o_r  characters  need c o n f l i c t  characters  same  intraspecific  ability  imposed  important be  (because  individuals).  characters  competition,  competitive  species  frequency  individuals).  Pimentel's  selection  i s more  (because  between  inter-  competition  Pimentel's  competition  versus is  i n both  competition  intra-  Competition  intraspecific  dependent  interspecific  of  ability.  intraspecific  effects  alterations  because  interspecific  niche is  genetic  with  , or  . important  in  interspecific.competit ion.  3) D i f f e r e n t  limiting  Another of  mechanism t h a t h a s been p r o p o s e d  more than  the  (Harper recent  one s i m i l a r  density  independent et  factors  of  factor, al  .  literature.  each  species  in close proximity  species  o r by t h e same 1961). This  idea  f o r the  is  limited  factor  Braakhekke was  at  by  occurs a  to  when  different  different  (1980) r e v i e w s  referred  'balance'  times  relevant  earlier  with  215  regard or  to  such  predator  otherwise all  superior  limiting is  rarely  of  one  and t h a t a  limiting case  mineral  size  (Sheikh  i n experiments with  when  both  species  were l i m i t e d dominant. species  At i n t e r m e d i t e was  coexistence It  species capable  limited  not  usually  (Grime  1979).  some s o i l s  by  a  nitrogen  other  f o r grasses  Similarly  be b e s t  the  root  p o r e s of a c e r t a i n in  adjacent  pores  Tilman  (1977)  1969)".  algae  that  by p h o s p h a t e , one s p e c i e s  the second  whereas when b o t h  species  was  was  species  competitively  o f p h o s p h a t e and s i l i c a t e , different  is  Grubb  1969); on  mineral  in soil  the other,  levels  as  b u t p h o s p h o r o u s and  (Thurston  & Rutter  where  resource  and  each  stable  occurred.  i s apparent  interpreted species  "...on  1963).  limited  over  by s i l i c a t e ,  are  two s p e c i e s o f f r e s h w a t e r  were  dominant  d e n s i t i e s of  t h i s mechanism.  limiting  s p e c i e s may  found  suppress  factors operating  under  may be b e s t  of another  may  occurs  f o r grasses,  (Willis  species  pathogen  t o draw a t t e n t i o n t o i n s t a n c e s  the  sedges  different  competitively  ever  t h e g r o w t h of legumes  for  factors  resources  p h o s p h o r u s may be t h e p r i m a r y  growth  of  be  limiting  limit  nitrogen  size  to  tolerance,  o r keep p o p u l a t i o n  the f o l l o w i n g examples:  the primary  and  however  purported  soils  competitors  are the d i f f e r e n t i a l  potassium  Such  low enough so t h a t  necessary  (1977) g i v e s is  etc.  and hence c o m p e t i t i o n  resources often  f a c t o r s as drought  resistence,  competitors  It  limiting  in  terms  i s limited then  that  the  of s u c c e s s ,  this mechanismis of  niche  much more niche even  by  space  at  least  differences. one  factor  within  which  partially  Clearly,  than each  is  i f one another  species i s  i n t h e a b s e n c e of c o m p e t i t o r s ,  will  be  216  different.  For  example,  community  while  legumes  symbiotic  nitrogen  different  source  In occurs  explanation. will  fixing  particular  size  different  limiting  a  hinges  on  two  combining  ability.  for  both  the  ..fact  this  limiting  'Different by  van  niche  den  the  the  level  competition  limitation species.  is  habitat, this  competitive  one)  species not  the  by  two  i s not  in  Coexistence  abilities. for  For  one  r e q u i r e d by have  of both  competitive  is a better be  of  If  limited  coexistence.  competitor  possible in spite  habitat, features  f o r two  (1978) as sense  species  a form  a  of  different  for  both  amounts of  each  'functional'  (1927).  In  species  has  resources,  each  Johansen  d i c t a t e s that  is interpreted  of  of E l t o n  t o mean t h a t  Andrew and  a v a i l a b l e supply  or  same  will  the  i s taken  by  a  species.  in  using different  have  habitat  s p e c i e s would  factors'  capable  review  the  two of  where  a better competitor  particular  saturation  with  be not  different  extensive  for  balanced  Bergh & Braakhekke  above example, t h i s  have  in a p a r t i c u l a r  richness  in a given  coexistence  limiting  latter  another  species  f o r each  in a  requires  two  I f one  differentiation  of  environment  condition  the  resource  coexistence  of  sense the  resources, that  units,  particular  s p e c i e s might  In  nitrogen  legumes  however,  a  t h e c o n d i t i o n of  species.  the  for  (but  by the  space  resources  resources  Here,  of  each  sufficient  example, e a c h the  of  because  degree  resources  population  itself  niche  the  not  limited  for nitrogen.  prevailing  determine  be  bacteria.  same r e s o u r c e The  may  may  (or n i c h e )  overlapping f o r the  grasses  one  i.e.  resource 1978).  (e.g. This  resource  is  the a  i t is see  combined limiting  217  for  one s p e c i e s and t h e o t h e r  species.  I f however t h e y  resource,  any p o t e n t i a l  any  differential  resource  units to  different by  limiting  overall  competitive  selection  are  limited  other,  ability  same  but not i n v a r i a b l e ,  Chapter  1) ( p e r h a p s  that  simply  Coexistence  to the other.  same  the here  b u t by  balanced balanced as w e l l  when b o t h  species  suite, of a t t r i b u t e s  i n each  of  just  species  from t h e o t h e r )  of these  occurrence  Furthermore,  ability  each  of  i s not e x p l a i n e d  differentiation,  combining  units  be measured a g a i n s t  c a n a l s o be m a i n t a i n e d  different  t o t h e same e x t e n t  resource  not  resource;  certain,  reduce,  the  f o r the other  the a v a i l a b i l i t y  i n t h e same n i c h e .  for competitive by t h e  exploiting  t o reduce  factors.  abilities  is limiting  f o r c o e x i s t e n c e must  any ( e . g . f u n c t i o n a l ) n i c h e  competitive  by  are  capacity the  resource  maintains  a  (e.g. Table  1,  that permit  i t to  species, the a v a i l a b i l i t y  of  218  NICHE DIFFERENTIATION VERSUS COEVOLUTION OF ALTERNATIVE EVOLUTIONARY SOLUTIONS FOR  Selection  for  competitive  mechanism by  which c o m p e t i t o r s  Unlike  mechanism  the  selection in  an  the  of  escape  on  and  thus  must  preserved  natural selection  satisfy  This  coevolution,  two  follows  e.g.  (and  as  in  both  p o p u l a t i o n s must  interacting  reciprocal  selection  (1974) and  Levin  of  b a s e d on to  niche  (or  Maynard  Smith  1976,  Connell  1980).  According  to  Slobodkin  in a particular  ecological  differences studies  distinction  i t can  models  therefore contexts  response 1980).  of  to  context  feedback.  the  coevolution  displacement 1976,  (e.g.  Case  "if  be  as a x i o m a t i c  a  of  competitive  genetic  taken  of  Leon  p.122),  exist  become  species coexistence.  is  (Janzen  Roughgarden  must  is  in  have r e g a r d e d  (1961,  region  interaction  coevolution in this  character)  between s p e c i e s has of  each other  others  of  host/parasite,  adjusted  (1968) model of  context,  The  between  genetic constitution  frequency-dependent  as  &  persist  on  be  results  process  in a l l c l a s s i c  The  (1977) use  Pimentel's  this  Lawlor  for  to  competitors  some  & Udovic  reference  Contrary  pressure  b)  where n a t u r a l  a  a)  a  co-evolve.  interaction  predator/prey,  etc..  to  coexistence  tradition  represents  competitor)  criteria:  plant,  balance  weaker  coexistence,  pollinator/host  with  considered  differentiation  the  allows  coevolution  implied).  ability  from - or e l i m i n a t i o n of - t h e  species  by  be  niche  (operating largely  SPECIES COEXISTENCE  combining  can  COMPETITORS;  two  between universal  1979,  species  them".  that The  touchstone  Investigations repeatedly  219  and  almost  automatically  interpretation  involving,  between c o e x i s t i n g remarkable  to  competitively  entail some  competitors  have  found  excluded  of  the  reflective  of the inadequate  one e n t i r e (e.g.  (Hutchinson  individuals this  view  and t h e r e  (Antonovics  taxonomically ability  selection  pitted and  to their  against  a  collection  1976a, Raven  in  inferior  and s u p e r i o r g e n o t y p e s  present  data  showing  reciprocal  natural  genotype p a i r s of L o l i u m  (Figs.  18 and 19, C h a p t e r  relative  of  competitive  resources selection by  'genotype  abilities  (regardless into  dynamic  a taxonomic  of  a common p o o l  'attribute basis.  Although combining  variation in  The c r u c i a l  ability  at different  times  taxonomic  genotype', with  and  genotype' of  demonstration  appears  to  of t h i s The  contested  converge  strategies  which cannot  To t h e t a x o n o m i s t  repens  specialization. for  The  different  Trifolium  individuals  of a l t e r n a t i v e  both  type.  i n t e r a c t i o n s between perenne  contest  i s not 'species  'genotype a g a i n s t  species)  complexes'  1982).  of g e n e t i c  6) i s t h e f i r s t versus  different  competitive  from e i t h e r  unit  by some t o change  1976, H a r p e r  s p e c i e s ' , but r a t h e r  thinking  distinctive  interest  combining  any g e n o t y p e may be c o n f r o n t e d  phenomenon  Traditional  relationship.  for competitive  classic  view o f t h e s p e c i e s  two s p e c i e s w i t h  competitive  the  of e c o l o g i c a l l y  dynamic c o n t i n u u m  has not  is  embedded.  i s much c u r r e n t  distinct,  represent  respect in  ranging  in  how  This  t h e s p e c i e s a s more of a t a x o n o m i c a l l y wide  some  1961)).  treats  a  were  population  as  ecologic  as  an  i t  though  typological is  or  as  which  than  ideology  for,  d i f f e r e n c e i n niche  other  plankton"  search  'important'  that  the  "paradox  a  by  defined  be c a t e g o r i z e d on  or biogeographer  concerned  220  with  community  diversity with  composition,  here.  the s p e c i e s  i s the r e l e v a n t u n i t  To t h e e v o l u t i o n a r y e c o l o g i s t however  community  processes  and  dynamics,  of  concerned  the o p e r a t i o n a l  unit  becomes t h e g e n o t y p e and t h e p o p u l a t i o n  and community  into a single  F i g . 9, C h a p t e r 3 ) .  The  question  certainly niche  e v o l u t i o n a r y arena  valid,  features  same  habitat  combining  (c.f.  This process  in  a  shifts  in  gene  of  'tracking  frequencies,  each is  Pimentel  1961, R o s e n z w e i g  (Pimentel  et  systems. overlap  Raven al  1964, .  Because i n niche  1963), a  requirements  competitive  interaction  species  that  (however  coexistence.  The  'paradox'  little  apparent  differentiation  permits  that  is  (coevolution). involving  unlike  or  that i n  pollinator  (Sheppard  1975)  i s preserved,  may a l s o be p r e s e r v e d ;  their  niche  species  with  1975), h o s t / p a r a s i t e  model/mimic  between  to  i n predator/prey (e.g.  & Raven  need t o assume any ambiguous c o n j e c t u r e niche  not  and  requirements  the  potential  other'  plant/herbivor  Gilbert  to  variants  selection  f o r the e v o l u t i o n of c o e x i s t e n c e  &  the  they  for competitive  interaction  theories  (Ehrlich  adapted  i n each  preserved  but i t  requires that  genetic  ability  1973),  priori,  both  (or  reciprocal  competitors  a  In s e l e c t i o n  relevant  persistent  of  them  presence  competitive  coexistence by  render  is  some d i f f e r e n c e i n  coexistence  1982).  generate)  competitors  that  i s not only  their  Harper  relative  maintained  mutual  that  the  between  insistence  i n common t h a t  continuously  stable  fact  ability,  challenging  the  i s the cause  the  have  coexistence  but  or o t h e r w i s e  contradicts  a  of  (i.e.  a r e welded  there  i s no  some d i f f e r e n c e s u b t l e ) must  in  explain  of c o i n c i d e n t s p e c i e s need n o t be a  any  paradox  with at  221  all.  Moreover,  provides  selection  reconciliation  for  competitive  of  the  "convergent  adapatation"  adaptation"  t o o t h e r members of  competitive plants,  combining  one  might from  in  communities  is  older  (Table  12,  It for  Chapter  whenever  ability there  party  (Pianka  them  reasonable  in contexts  of  by  i f the  important  with  of  the  a niche  consequence  time,  i s always  i.e.  9).  'another' since  This  sampling  expected  theory  between  do  so.  This  assumes  environmental-  seems  involving  intuitively  interspecific  shift  however niche  'exploitation'  and  not  to expect  that  i f 'another'  2)  type  may  to  an  not for  interaction.  be  as  plants  competition and,  niche  consider,  be  inferior (of  any  is available There are  especially  readily do  to  intraspecific  a h i g h l y superior competitor  resources  partitionable;  and  other  interspecific  i s r e l e v a n t g e n e t i c and to  advantageous  to a v o i d the  Traditional  i s a reduced  qualifications 1)  of  significant  contact  interaction  competition  when f a c e d w i t h  animals,  readily  temporary  from  "...it  It i s also reasonable  would e v o l v e  for  i n a community  since i t i s possible for interference effects  competitors.  plants:  data  i n t e r s p e c i f i c ., c o m p e t i t o r s  competitor,  If selection  species with  1976).  i s t o be  whenever t h e r e  interference  fewer  "divergent  i n t h e model of F i g u r e  in a competitive  permitting  and  t o be  present  variation  type)  community.  and  3).  possible"  competitors  between  were i m p o r t a n t  ( s t e p 3A  the  t h a t .niche d i v e r g e n c e  felt  the  i s commonly b e l i e v e d t h a t ,  either  paradox  apparent  random a s s o c i a t i o n of  s u b s t a n t i a t e d by  ability  t o a common e n v i r o n m e n t  expect  deviations  combining  not  i s often  two with  available appear  as as  predominantly  i n c o n t r a s t to i n t e r f e r e n c e  222  effects,  exploitation  and  intra-specific  if  competitive  competitive The  combining  selection),  very  there  unequivocally populations, became)  even  such  plant  its  individual  Individuals  of one  that  exploited  i s not  leave  any  more  exploiting  the  expansion  for  necessarily species  require  A here  then  has  favour  competitive  coincidentally  or  selection  divergence  after should  were  (or  reason  for  this  follows  forces operating  which  in this  fitness  than  on  with  case  interspecific  B.  would  than The  presence nothing  individuals  not  those  different  'find'  be  a  niche  expected  individuals  of  t o do  B,  B.  and Any  in  competition.  to  of  c o n s e q u e n c e would be from  both  intraspecific  i s no  A w h i c h happen t o  species B  from  were t o t a k e p l a c e ,  have t o c o n t e n d  divergence  the  may  opportunities for divergence  same n i c h e as not  that,  two  descendents  A,  is  the  s p e c i e s , say by  inter-  between  still  competition  1)  exploitation  If niche divergence  exploitation  both  differentiation.  t h a t subsequent  selective  p o p u l a t i o n s would  on  if  niche  The  the  by  c o n s e q u e n c e of  (either  reason  i f such  instances.  effect  that  to  abundant. of  is  i s no  The  felt  i n s t e a d of n i c h e  similar  lead  consideration in  2)  a l w a y s be  i s threatening, selection  ability  of  are  will  competitors.  exclusion  consequence  abilities  effects  A  niche  does not  even  advantage  for  with avoidance  of  species  the  above  B. A  question  considerations competitors precisely  which is  coexist? this  Intraspecific  fact  presents  the The  itself  following: answer  i s almost  that presents  competitors  coexist  How  by  from do trivial  intraspecific and  it  is  far-reaching implications. and  l a r g e because  relative  223  competitive the  abilities  members of  members  An  and  not  individual  will  b e t t e r at  leave  taxonomy  of  mechanism however explanations individual species  of can  in  a  competitive  broadly  divergence  we  i f these  divergence  populations with.  Since,  specific effects, severity  expect  units; its  an own for  application  as  , irrespective  of  two  largely  in  by  species  have  'exploitation'  selection  for  s p e c i e s have c o m p e t i t i v e  combining  ability.  i n both  little  populations, abilities  divergence  competitive One  relevant  strong  advantage . because  behave c o m p e t i t i v e l y as  niche  population.  not  former  Selection  broader  that  compete  The  members of  individuals  those  of  taxonomic  environment.  suppose  and  affords  competitive  of  i f we  niches  should  of  if  regardless  only  with  however has  coexistence  overlapping  of d i f f e r e n t  limited  ability  Accordingly,  mechanisms,  Niche  the  is  their  or  contestants. it  another  interaction'  interaction'  as  The  within  descendents,  avoid competition  resource  combining  explains  taxonomy.  coexistence  from  'balance'  more  potential  is restrictive  rarely  to diverge  a  amongst  selection.  ' avoiding competitive  their  the  balanced  natural  'decide'  ' more s u c c e s s f u l i n c o m p e t i t i v e  the  it  do  roughly  through  in contrast, maintain  descendents are are  maintained  population  of a p o p u l a t i o n  population, own.  the  are  would  are  would  pressures no  such p o p u l a t i o n s  i n response  one  would e x p e c t  divergence  to  intraspecific  a  'single  overall virtually do on  more e x p e c t to  within either  competition.  and  equivalent to  individuals divergence  interspecific  the  species' to  inter-  nothing  niche  begin intra-  in  their  change in  between  in  the  either  competition  population  two  two than  response  224  Theoretical replacement van  den  Berghe & B r a a k h e k k e  dependent  of  differences. overall  It  niche  two  species  be  noted.  to  (Berendse  environment  respond  and of  roughly the  the  produces  of by  the  response  artificial reflect  environmental  the  against  of  ways t h a t the  inter-  that  specific combining  in species  with  competitive  a  stochastic  environment,  of e x t i n c t i o n of point  occurs  occur in  the  if  fertilizer species  Extinction  v a r i a b l e b a c k g r o u n d of  in  the  competitive  ability.  can  Pigott  in v e g e t a t i o n most  application respond  must  competitor  but  the  inferior  changes  exclusion  changes,  of  however  relative  combining  exhibit  one  when an  i f the weaker  for competitive  treatments  a l l of  environment in  to  or  will  (1982) d i s c u s s e s examples of c o m p e t i t i v e in  niche  populations  may  not  frequency  d i f f e r e n c e between  a much s u p e r i o r c o m p e t i t o r .  changes  stable  reflect  balanced  A qualifying  s p e c i e s , but  selection  under  1970  two  risk  exclusion  a  competitive  held notion  1981b).  i s faced with  competitive  abilities  with  fluctuations in  increases  intra-  E x t i n c t i o n from c o m p e t i t i o n  competitor due  commonly  frequencies  strongly  from  no  1961,  that  to  need be  species  differentiation  unpredictable  which  experienced  a  shown  thought  substitutive  1960,  p o s s i b l e only  contrast, there  is  the  is  the  (de Wit  have  usually  in coexisting  abilities, large  In  1978)  species  hardships  competitors  little  two  regulation,  ability.  using  s e r i e s model of c o m p e t i t i o n  equilibrium  the  considerations  to  and  their  stochastic  involve do  not  biotic events  nature. A  fully  adequate  requires a greater  understanding  e m p h a s i s on  the  of  species  variability  coexistence  within  species  225  with  respect  competitive  to both  fundamental  abilities,  concurrent  differences  between s p e c i e s .  ability  competitive  in  and  concert  natural in  in nature  selection  Chapter  3  components and  other.  Whether  21-B)  or  C)  both  or  of as  two per  four  upon  Efforts  are  21-A)  combining  to  operate  evolution  one  combining  route  proposed  e a c h of  ability  ability  exclusion  and these  more t h a n  l e a d i n g to  will  the  c o n t r i b u t i o n of  affects  combining  (or whether c o m p e t i t i v e - Fig.  the  to a f f e c t  for ecological  evolutionary  likely  of  needed t o r e v e a l how  natural selection  for competitive  Gause's p r i n c i p l e  awareness  are  f o r community  relative  the  (Fig.  (Fig.  21-  coexistence  i s the  depend on  outcome  at  least  factors:  1) The in  an  describe  when i t i s l i k e l y  d e p i c t s the  species  together  9).  selection  selection  with  and  Selection for ecological  i n t h e model  under what c i r c u m s t a n c e s two  requirements  combining a b i l i t y and  (Fig.  niche  initial  both  m a g n i t u d e of d i f f e r e n c e (or e x t e n t  fundamental niche  first  encounter  .  requirements  For  example,  species overlap  e x t e n s i v e l y and  than  then  the  other,  be  one  exclusion  will  virtually  competitive  combining  differences  in competitive  ability  is  if  is a  i t i s reasonable  and  the  to expect  are  niches  far superior  unlikely  similarity)  competitive  immediate.  abilities  of  in  large.  that  ability of  competitor competitive  Selection cases  two  in  for which  226  2)  The  availability  'relevant'  genetic  requirements may  This  variants  versus  than  relative  potential  will  be l a r g e l y  species  to  competitive  ability.  generating biotic  species w i l l adaptive  environment  to relative  by t h e n a t u r e  and m a g n i t u d e o f  apomixis,  g e n e r a l l y have  will  & Tilman  resource  by a m a t r i x  1979).  particularly  to  in  available  expansion  plants  sufficiency  of r e s o u r c e s  resources  which  partitionable  units  neighbours  spread).  potential  or  asexual  in  the  or d i s p l a c e m e n t  are  may  .  is  species  Novel  mostly  in their  largely  not  favour  circumstances.  virtually  (Harper  i n the  prevailing  competitors  instead  which, being  not  for  niche  h a b i t a t o r s p e c i e s may  of nearby p o t e n t i a l  such  two  1978).  space  Selection  ability  versa.  i n the  clonal  greater  incrossing  be u n a v a i l a b l e i n a c e r t a i n  combining  competitive  and v i c e  o p t i o n of " e v o l v i n g - t o - a v o i d - c o m p e t i t i o n "  (Krebs  Some s p e c i e s  requirements,  allogamy,  than  .for niche  constrained  niche  g e n e t i c v a r i a n t s and t r a c k i n g c h a n g e s  1976, G l e s e n e r  3) The r a n g e o f  s p a c e may  determined  autogamy,  environment  (Solbrig  related  different  fundamental  gene p o o l and t h e means o f p r o p a g a t i o n  (i.e.  Outcrossing  to generate)  pertaining  t o fundamental niche  the a v a i l a b l e  that  (or  have more g e n e t i c v a r i a t i o n  ability  the  of  be  so t h a t feasible  competitive  This . applies sessile,  immediate  packaged  in  1965, 1968), and t h e same  a r e a l s o f o r c e d t o make demands o n .  require a vicinity, discrete resources  227  4)  The  presence  concomitantly difficult compete sense Story  (e.g.  (but  Agnew  Haines  Putwain  & Harper  competitor  may  interact  1970,  stand  to  result.  by  16,  Chapter  niche  selective  5;  Chapter  for  successful  descendents  and  reduction selection  a l l .  selection  competitive  to  this ability in  'thrum'  the  occur  offer.  competitive  favoured  (and  i n the  may  i f beneficence is  is  r e l e v a n t to Lolium  present  the  perenne  study  may  (e.g.  But  relation  (Fig.  in  inferior  aggressiveness  of  combining  order  compete  to out  equivalent only  an  form, but the  and  with they  leave that  also  any  i n such  competitive  improvement  superior  ability  be  'male', and  to r e p r o d u c t i o n  points  more not  will  will  in dioecious  p r i m r o s e ) the  (1961)  involve  for competitive  example,  in regards  favour  the  For  population  in primrose)  Mather  will  of  run i f  stable coexistence  outbreeding  species  cooperative  at  be  This  mating.  for resources.  require a  force.  within a single  each  power  long  A  6).  ' p i n ' and  other  and  demonstrated  or h e t e r o s t y l i c  instances  i n the  therefore  1964,  1972).  something p o s i t i v e  may  may  another  Harper .  i n t e r a c t i o n s between  'female''(or  will  al  'loose out'  c o n v e r g e n c e may  repens  Cooperation  species,  in  1961,  Walker et  selection  of c o m p e t i t i v e  Trifolium  necessary  balanced  natural  enough  explanation and  roughly  Moreover  strong  Mather  I t i s not  individuals  or m u t u a l i s t i c a s s o c i a t i o n s i n w h i c h  are  maintained)  that  .  beneficially  1960,  e l i m i n a t e s a s p e c i e s w h i c h has  abilities  species  often overlooked) yet  &  relationships occurring  between two  s e n s e and  Commensalistic  a  competition  imagine  i n one  superior it  with  to  1967,  ( o r a b s e n c e ) of p o s i t i v e  a  form.  in  relative This  c o u l d occur  is not  228  only  within  different and  yet  a  species  another  species  plant  Furthermore, not  a)  b)  nitrogen  predators  c)  (Harper  of  benefit  6).  (enhancing) e f f e c t  number  benefit  of  possible  gained  gained  in plants  protection  & Sagar  by  by  on  ways.  one  species  i t s associate.  include: (e.g.  a  from c l i m a t e  1953), p e s t s and  support  (Trenbath  f o r growth  legume  may  (Marshall  diseases  1967),  (Atsatt  &  humidity)  (e.g.  epiphytes)  or  1976).  s p e c i a l environmental favourable  for  conditions  (e.g.  germination  and  shade  or  establishment  1964).  Encouragement  discouragement f) L i b e r a t i o n g)  the  two  non-legume).  physical  lodging  Providing  (Harper e)  a  between  b e n e f i c i a l manner  Chapter  in l i m i t e d supply  physical  Providing  high  to a  (e.g.  a positive  in  also  1976).  against d)  exert  materials  Providing  O'Dowd  may  beneficial interactions  Providing  provide  resources  same k i n d  above but  i n some o t h e r  for  species  the  as  interact  i n a mutualism,  be  Possible  species  which  a l s o compete  A plant  may  single  Dispersal  destruction  by  of  of  favourable  detrimental  of  stimulating with  man  by  ones  rhizosphere  ( C h r i s t i e et a l  chemicals  commercial mimicing  the  components.  (Roy  seeds  1960, and  crop plant  .  1974).  Tukey  1970).  protection (Wickler  or  from  1968).  229  CONCLUSIONS  Difference  i n niche  some s p e c i e s a r e a b l e studies  on a n i m a l s  1978),  tend  Darwinism  to  to  that  this  origin  to coexist.  that,  between  tradition  communities.  virtually ecologists  nothing are  derivative  et a l .  and  especially  in  contrast  comparable  to  the  coexistence.  have a t t e m p t e d nature, Grubb  to  largely  largely  unable  account  on  provides  an  evolutionary  mediated  a coevolutionary  theory  To  insist  t h e enormous types  animals,  Plant  in  explaining  some p l a n t e c o l o g i s t s coexistence  in  d i f f e r e n t i a t i o n (e.g.  1978, Newman '1982). combining  s p e c i e s occupy a  ability  which  does  different  frequency-dependent  i n t e r f e r e n c e phenomena.  selection  of have  niche'.  for coexistence  of c o m p e t i t i v e  in  t o a d o p t most o f t h e  i t depend on  by b e h a v i o u r a l  consequence of r e c i p r o c a l persistently  every  species  many  to  species  of niche  mechanism  n o r does  of  i s 'Gaussian'  in  for competitive  require that  i n a community,  selection  are  plant  basis  other  adequate  of t h i s ,  for  by s e l e c t i o n  unequivocally  niche  the  spite  1977, van den Bergh'& B r a a k h e k k e Coevolution  not  In  from  1974, Diamond  310).  'food  m o d e l s and t h e o r i e s w h i c h a r e a p p a r e n t l y animal  of  contradicts  Plants,  therefore  mostly  hypothesis  1952, p .  patently found  e x p l a i n why  of a s p e c i e s , mechanisms w h i c h  o f law o r axiom however  amount o f s p e c i e s d i v e r s i t y plant  evidence,  i t and p o p u l a t i o n s  (Gilbert  i s some s o r t  certainly  1974, S c h o e n e r  a  in a population  persist"  and  support  will  Much  ( e . g . s e e Cody  "...  reduce c o m p e t i t i o n tend  requirements  coexistence  Iti s  i n which  means t h a t d i f f e r e n t  making demands f o r t h e same l i m i t i n g  the  species  resources.  230  Moreover, reason  given  sufficient  to suspect the  relative  inferior  plant competitors  more  (or  dismissed)  because have  adjusting  a  i t has been  have  the  experimental  finding  that  competitive maintained combining do  ecology this  the  two  pressures by r e c i p r o c a l  ability.  ability  i n the region  selection be  for ecological  avoided.  completely  'Winning describe  it/losing only  a  uniform Gause's  was n o t u n i f o r m .  exert other they  claim;  relatively and  that  have  the a b i l i t y for  the This  rather i t equal this i s  competitive,  i f two s p e c i e s have  ability  niches  t o c o e x i s t may be  competitive  combining  where c o m p e t i t i o n c a n  i t ' or ' a v o i d i n g  alternatives  outcome; e v o l u t i o n by n a t u r a l s e l e c t i o n a way a s t o p e r m i t  in  appear  o v e r l a p as i t i s a c o n s e q u e n c e o f  combining  the  ability'  .  Slobodkin's  even  selection  of niche  coexistence  does n o t d i s p r o v e  each  overlap,  as much a c o n s e q u e n c e o f  species  of n a t u r a l s e l e c t i o n i n  selection,, i . e .  not c o m p l e t e l y  overlooked  together  may  be  niche  t h e o r g a n i s m s do n o t have  species  Furthermore,  for  'competitive  environment  -against  may  i f two s p e c i e s w h i c h  finding  does n o t even p r o v e the  1)  been  a priori  persist  i t proves that that  for  and t h e r o l e  that  has  of  underestimated  same  or  in fact  suggests  distinction  (1968) r e a s o n e d  rather  ecology  theory  to  i n a p o p u l a t i o n of  1, C h a p t e r  idea  i s good  selection  opportunities  This  viable  environment,  hypothesis;  which  as  Table  there  natural  ability  the  in vegetation.  misrepresented  Slobodkin  same  ( e.g.  than  t h e meaning and  been  to  competitive  numerous  differentiation  variation,  that o p p o r t u n i t i e s f o r  improve  far  genetic  for  i t ' do n o t competitive  may a l s o p r o c e e d  a ' c o n t i n u a t i o n of the match'.  i n such  231  LITERATURE  CITED  A a r s s e n , L.W., R. 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